Methods for chemical analysis of soils

LIBRARY LANDCARE RESEARCH NZ P.O. BOX 69, LINCOLN, N.Z.

METHODS FOR CHEMICAL ANALYSIS OF SOILS

L. C. Blakemore, P. L. Searle, B. K. Daly NZ Soil Bureau, Lower Hutt

NZ Soil Bureau Scientific Report 80

NZ Soil Bureau Department of Scientific and Industrial Research

Lower Hutt, New Zealand 1987

Bibliographic Reference: BLAKEMORE, L. C.; SEARLE, P. L.; DALY, B. K. 1987: Methods for Chemical Analysis of Soils. NZ Soil Bureau Scientific Report 80. 103 p.

ISSN 0304-1735

(Previous editions published as NZ Soil Bureau Scientific Report 1 OA)

Editing: David Isaacs Typing: Tessa Roach Draughting: Carolyn Powell

CONTENTS

INTRODUCTION

MOISTURE FACTOR

2 SOIL pH 2A pH in H20 2B pH in 0.01 M CaCl, solution 2C pH in I M KC! solution 2D pH in NaF solution 2E pH in H20 2

References ..

3 CARBON 3A Total carbon using high-frequency induction furnace 3B Colorimetric determination of organic carbon 3C Pyrophosphate-extractable carbon References ..

4 NITROGEN

LIBRARY LANDCARE RE~EAFl©Fi NZ P.O. BOX 69, l.INGGLN, N.Z.

4A Total nitrogen where nitrate is present in trace amounts 4A.I Manual method 4A.II AutoAnalyzer 4B Total nitrogen where nitrate content is high 4B.I Manual method 4B.II AutoAnalyzer 4C 2 M KCl-extractable nitrate References

S PHOSPHORUS SA Truog-soluble phosphorus SA.I Manual method SA.II AutoAnalyzer SB Olsen-soluble phosphorus SC Bray 2-soluble phosphorus SD O.S M H,SO,-soluble phosphorus SD.I Manual method SD.11 AutoAnalyzer SE Organic phosphorus SE.I Manual method SE.II AutoAnalyzer SF Total phosphorus SF! Fusion with NaHCO, SFl.I Manual method SFl.11 AutoAnalyzer SF2 Fusion with NaOH SF2.I Manual method SF2.II AutoAnalyzer SG Phosphate retention References

6 CATION EXCHANGE PROPERTIES 6A Cation exchange properties determined by I M ammonium acetate (pH 7) 6AI Exchange methods 6A I a Macro leaching 6AI b Semi-micro leaching 6Alc Automatic extractor 6A2 Individual exchangeable bases 6A3 Total exchangeable bases (TEB) 6A3a By titration 6A3b By summation 6A4 Cation exchange capacity (CEC) M.4.1 Distillation 6A4.II AutoAnalyzer 6AS Calculation of% BS (pH 7)

s 7

9 11 11 l l l l 12 12

13 13 18 20 20

21 21 22 23 2S 2S 2S 27 29

31 32 32 33 3S 36 37 37 38 39 39 39 40 40 41 41 42 42 43 44 44

47 48 48 49 49 SI S3 SS SS SS S6 S6 S6 S7

6B Effective cation exchange capacity (ECEC) 6B I I M KCl~extractable aluminium 6B2 Calculation of ECEC 6B3 Calculation of% BS (ECEC) 6C Cation exchange capacity at pH 8.2 6Cl Exchangeable acidity 6C2 Calculation of CEC at pH 8.2 6C3 Calculation of% BS at pH 8.2 6D Cation exchange properties determined by 0.01 M silver thiourea 6D I Extraction 6D2 Individual exchangeable bases 6D3 Cation exchange capacity .. 6D4 Exchangeable Al and Mn 6D5 Calculation of effective CEC 6D6 Calculation of% BS(AgTU) References

7 RESERVE NUTRIENTS 7 A Reserve potassium (KJ 7B Reserve magnesium (Mg,) References ..

8 EXTRACTABLE IRON, ALUMINIUM AND SILICON SA Acid oxalate-extractable iron, aluminium and silicon 8A I Extraction by shaking .. 8A2 Extraction by leaching .. 8B Pyrophosphate-extractable iron and aluminium SC Dithionite-citrate-extractable iron and aluminium References

9 SOLUBLE SALTS .. 9A Water extraction-1:5 sOil:water ratio 9Al Conductivity .. 9A2 Water-soluble basic cations 9A3 Water-soluble sulphate 9A3.l Distillation 9A3.ll AutoAnalyzer 9A4 Water-soluble chloride References

lO CALCIUM CARBONATE l OA Gasometric carbon dioxide method IOB Differential method IOC Weight-loss method !OD Correcting ammomium acetate-exchangeable Ca for CaCO,-Ca References

II SULPHUR I IA Extractable sulphur I !Al Phosphate-extractable sulphur l IA2 Phosphate-extractable sulphate l IA2.l Distillation l l Al.II AutoAnalyzer References

12 ANALYSIS OF PLANT LITTERS AND PEATS 12A Loss on ignition . . . . . . . . 12B Preparation of solution for analysis and determination of acid-insoluble ash l 2C Total phosphorus l 2D Total bases .. References

13 REPORTING AND RATING OF RESULTS

58 58 59 59 60 60 61 61 63 63 63 64 65 66 66 66

67 67 69 70

71 71 72 73 74 75 76

77 78 79 80 81 81 81 82 82

83 83 84 86 87 87

89 89 90 94 94 94 96

97 97 98 99

100 100

IOI

5

INTRODUCTION The first comprehensive account of the chemical analytical methods that were being used by NZ Soil Bureau, 'Methods of Chemical Analysis for Soil Survey Samples' by A. J. Metson (NZ Soil Bureau Bulletin 12), was published in 1956. This provided background information on the purpose of the determinations and on the technical details of the methods.

In the following years, soil chemistry made tremendous advances and many new analytical techniques were developed which improved the speed and accuracy of standard analytical procedures and facilitated measurement of a wider range of properties.

These developments were incorporated in the first edition of NZ Soil Bureau Scientific Report JOA, published in 1972. This publication was written with a minimum amount of explanatory information on the principles of the methods and application of the results. It described new methods or new techniques that had been developed since 1956, some of which required modem and expensive equipment.

The 1977 revised edition incorporated improvements in instrumental methods developed within the Soil Analysis Section at Soil Bureau. Also, some changes to procedures, and to the ratings for chemical properties were made. Most concentrations were changed to SI units, with the exception of cation exchange properties, where the expression milliequivalents was retained.

The second revision (1981) included several changes to techniques and terminology. Some were relatively minor, but those concerning determination of extractable sulphur and oxalate-extractable Al, Fe and Si were quite major. The use of 'Soil Taxonomy' for classification of soils by Soil Bureau, on a trial basis, made it necessary for several additional analyses to be done. These were pHN•F• KCl-extractable Al, exchange acidity, dithionite-citrate Fe and Al, and pyrophosphate-extractable Fe and AL Details of methods for these determinations were included.

This new publication, which now replaces NZ Soil Bureau Scientific Report JOA, includes minor changes to existing methods and the addition of the following methods:

Pyrophosphate-extractable carbon 2 M KCl-extractable nitrate Olsen-soluble phosphorus Bray 2-soluble phosphorus Total phosphorus (NaOH fusion) Leaching by automatic extractor (for cation exchange properties) Cation exchange properties determined by silver thiourea extraction Phosphate-extractable sulphate determined by AutoAnalyzer.

In addition, a new system of numbering the methods has been used. The first number indicates a property or group of properties; the other letters and numbers before the decimal point indicate different methods to measure the property, or different forms of it; and the roman numerals after the decimal point indicate the type of method, manual or automated. Thus,

5 indicates Phosphorus 5F indicates Total phosphorus

5F2 indicates Total phosphorus by fusion with NaOH 5F2.1 indicates Total phosphorus by fusion with NaOH, manual method.

6

The use of milliequivalents* has been retained for cation exchange and reserve nutrient data. However, the term me.% has been replaced by the more correct me./100 g. Similarly, some phosphorus fractions are now .expressed as mg/100 g instead of mg %.

Because there are now several methods by which pure water may be produced, the term 'distilled' has been deleted and for the purpose of these methods 'water'. means pure water. A further change involves the deletion of reference to the purity of reagents to be used (e.g., A.R.). This has been done because it may be assumed that pure reagents are necessary for use in these methods unless otherwise stated.

In these methods, results on blanks are usually subtracted from the results on samples, after the final calculation. Where samples require extra dilution, blank solutions should be similarly diluted. This allows for the situation where the main. source of contamination is the diluent solution rather than a reagent. *l _ 1 mmol

me. - charge on free ion

7

1: MOISTURE FACTOR

Most results are reported on an oven-dry basis, but as oven drying causes changes in several chemical properties of soils, analyses are carried out on air-dried samples (i.e., dried at temperatures of no more than 30°C). In order to convert results to an oven-dry basis, a moisture factor is applied in the calculation of results.

PROCEDURE

Weigh accurately a 10-20 g sample of soil (air-dry, < 2 mm ) into a weighed dish with lid (a labelled, aluminium dish is recommended), dry in an oven at 105°C for 8-24 hours.

Remove from oven, fit lid, cool and reweigh (all weighings should be made to the nearest 10 mg). Because oven-dry soil picks up water from the atmosphere very rapidly (even in some desiccators), it is necessary to reweigh without delay, either with a top-loading balance as soon as the dish is cool enough to handle or after cooling in an efficient desiccator.

CALCULATION OF RESULTS wt air-dry soil (g) . .

d .1

( ) ~ M01sture Factor (M.F.) (expressed to 3 decimal places) wt oven- ry s01 g

9

2: SOIL pH

The measurement of soil pH is usually regarded as one of the simplest determinations used in soil chemistry. However, even a cursory inquiry into the factors involved will show that many complications and difficulties occur.

Several of these difficulties are caused by the fact that the pH theory applies to dilute solutions of simple electrolytes (Bates 1964). Although measurements may be made quite easily on soils and soil suspensions, there is considerable doubt about the significance of the results obtained in terms of hydrogen-ion activities. Available evidence indicates that the apparent pH of soil obtained by use of a glass electrode pH meter reflects hydrogen-ion activities in the bulk solution surrounding the electrodes, rather than hydrogen-ion activities in the ion atmosphere around the soil particles. However, the values obtained by carrying out pH measurements on soils do provide useful correlations with other soil properties, and it is important to choose suitable conditions for making the measurements and to realise the limitations of these conditions when interpreting results.

The choice of conditions for measurement of pH in soils involves the following factors:

Moistness of soil to be tested

Suspension medium

Ratio of soil to suspension medium

Degree of stirring

Positioning of electrodes

MOISTNESS OF SOIL TO BE TESTED. The choice is between air-dry soil (as prepared for chemical analysis) and soil in its natural state of moisture. The principal objection to using air-dry soil is that the drying process can cause changes in pH values (e.g., through oxidation of sulphides). The main objections to using naturally moist soil are the difficulties of weighing a representative sample of this material and the effects of biological activity on pH during storage of moist soil. Because of these problems we now routinely use air-dry soil at NZ Soil Bureau.

SUSPENSION MEDIUM. Ideally, measurement of soil pH should be made in soil in its natural condition, but for several reasons, e.g., fragility of the glass electrode, this is not possible and, in practice, it is usually necessary for the soil to be suspended in liquid to enable measurements to be taken.

In New Zealand laboratories, water has usually been the suspension medium for pH determination for both pedological and agricultural measurements. However, electrolyte solutions are often preferred because the results are more reproducible, they are not influenced by the salt content of the soil, and the values obtained are less dependent on the positioning of the electrodes. 1 M KC! and 0.01 M CaCl2 are the most commonly used solutions.

When I M KC! is used, extensive ion exchange takes place, including the release of aluminium. Hydronium and other proton. donors are brought into solution, influencing the glass electrode and lowering the measured pH (Black 1968). It is not to be expected that the pH values obtained with I M KC! will represent those

10

of soil solutions, but Black has suggested that they may approach the pH values in the ion atmospheres of the original soil.

Peech (1965) advocated use of0.01 M CaCI, as the suspension medium, because it is similar in electrolyte composition to soil solutions found at optimum moisture conditions for plant growth in 'non-saline' soils (i.e., soils of low salt concentration). He noted that this medium is independent of dilution over a wide range of soil:suspension medium ratios. At Soil Bureau, limited experience with 0.01 M CaCI, has shown that it has some advantage over water as a medium for measuring soil pH because results are more easily reproduced.

The overall effect of using salts is to lower the measured pH values. With KC! the effect varies considerably, and the pH is often more than I unit lower than results obtained with water. For New Zealand soils, results with CaCl2 are about 0.5 to I pH unit lower than with water.

RATIO OF SOIL TO SUSPENSION MEDIUM. Because soil pH values decrease with increasing electrolyte concentration, it is to be expected that the electrolyte effect would significantly change with different dilutions of soil with water. In practice it has been found that the dilution effect varies considerably from soil to soil, and in order to render measurements by different investigators comparable, the International Society of Soil Science committee on soil reaction measurements (I 930) adopted a standard dilution ratio of I part (by weight) of soil to 2.5 parts of water. This ratio was chosen for convenience of pH measurement in mineral soils. Other ratios have from time to time been recommended, but it has been decided at NZ Soil Bureau that standardisation is important and that the international 1:2.5 ratio should be used. However, in soils which are very high in organic matter· content (peats), it has been found necessary to use wider ratios (1:5 or 1:10) in order to obtain workable slurries.

DEGREE OF STIRRING. Because it has been found that pH levels for some soils vary with degree of stirring during preparation of soil suspensions, it has been made standard practice to stir the suspensions vigorously using a mechanical stirrer or homogeniser.

POSITIONING OF ELECTRODES. When water or 0.01 MCaCl2 is used as the suspension medium, the position of the reference electrode is most important. A potential difference is apparently set up at the junction of the soil and the KC! bridge. As the potential difference varies with the concentration and nature of the soil in the contact area, standardisation is better achieved by positioning the calomel electrode in the supernatant liquid.

Although the positioning of the glass electrode is not of such importance, it is preferable to place it in the sediment zone, where the system is more strongly buffered and hence less susceptible to minor disturbances such as carry-over from the previous sample.

The electrodes should be positioned so that the contact point of the references electrode is approximately l cm higher than the centrepoint of the sphere of the glass electrode. Combined electrodes are suitable, providing the bulb is positioned in the sediment when the reference outlet is in contact with the supernatant.

The pH meter should be adjusted so that correct pH readings are obtained for two buffer solutions of widely differing pH (e.g., pH 4.0 and 6.5).

PREPARATION OF REAGENTS pH 6.5 BUFFER: CONCENTRATED BUFFER. Dissolve 89.7 g .disodium

hydrogen phosphate (Na2HP0,.2H20) and I 58.4 g potassium dihydrogen phosphate (KH2PO,) in water. Make up to one litre with water.

WORKING BUFFER. Pipette 20 ml concentrated buffer into a 500-ml volumetric flask and make to volume with water:

/

JI

pH 4.0 BUFFER: 0.05 M POTASSIUM HYDROGEN PHTHALATE. Dissolve 5.106 g COOH.C,H,.COOK in water and make up to 500 ml with water.

0.01 M CALCIUM CHLORIDE. Dry CaCl2 (anhydrous) at l I0°C, dissolve 1.11 g in water and make to I litre.

IM POTASSIUM CHLORIDE. Dissolve 74.56 g KC! in water and make to I litre.

SODIUM FLUORIDE, saturated (approximately M). Add 4 litres of water to 180 g NaF, in a 4-litre plastic bottle. Shake well, and let stand for 2 days with occasional shaking. After excess NaF has settled, check that the pH is between 7 .2 and 8.1. Take 50-ml aliquot, heat to boiling, add 5 drops of 0.25% phenophthalein and titrate with 0.01 M NaOH to a pink end-point while hot. If the solution has a pH of more than 8.1 or if the titratable acidity exceeds 0.25 me. per litre (more than 1.25 ml 0.01 M NaOH for 50-ml aliquot), discard and try another source of NaF. This laboratory uses Fisons A.R. grade.

PROCEDURE Weigh 10 g of soil (air-dry, < 2 mm) into a 100-ml beaker and add 25 ml water.

Stir vigorously with a homogeniser or high-speed stirrer.

Leave to stand overnight.

NOTE: When wider soil:suspension medium ratios are needed to achieve a workable slurry (e.g., for peats), record the ratio used.

Thoroughly wash the electrodes with water.

Position the soil sample on the instrument so that both electrodes are well covered.

Without stirring, measure and record the pH.

Carry out duplicate determinations on separate subsamples. Replicate determinations should give results within 0.1 pH unit.

2B pH IN 0.01 M CaCI2 SOLUTION

PROCEDURE Carry out procedure as for 2A except that 25 ml 0.01 M CaC12 solution is added to the soil instead of H,O.

2C pH IN 1 M KCI SOLUTION

PROCEDURE Carry out procedure as for 2A except that 2'.r ml I M KC! solution is added to the soil instead of H,O.

2D pH IN NaF SOLUTION

This method is based on that of Fieldes and Perrott (1966) and is used as an indication of the presence of active aluminium. It depends on active aluminium sorbing fluoride ions with consequent release of hydroxyl ions. It is the basis of a 'Fluoride field test' in which a portion of soil is reacted with NaF. If'allophane'

12

is present, the resultant high pH turns phenolphthalein-soaked filter paper red. High pHN•F values are found in soils derived from volcanic ash and in the illuvial horizons of podzolised soils.

PROCEDURE Weigh I g soil (air-dry, < 2 mm) into a 100-ml beaker, add 50 ml NaF reagent and' stir vigorously for I min.

Place pH electrode/s in suspension and swirl gently.

Read pH exactly 2 min. after adding reagent, ensuring that the suspension is well stirred immediately prior to taking reading.

This method, which is adapted from that of Ford and Calvert (1970), is used to indicate the presence of oxidisable sulphides in soils and sediments. Its main use in New Zealand is with mining overburdens which may cause acidity problems when exposed to air.

The samples are heated with 30% hydrogen peroxide, and any reduction in pH caused by the conversion of sulphides to sulphuric acid is measured. If the pH is 3 or less, problems with acidity in the materials sampled can be expected.

PROCEDURE Weigh I g soil (usually wet soil as received) into a 200-ml, wide-mouth conical flask.

Add 20 ml 30% H20 2 and place on a boiling waterbath. Remove from the waterbath if the reaction is too vigorous.

Continue heating until reaction ceases and solution clears, which should take about 30 minutes. Organic soils may require the addition of further H20 2 before they clear. It is important to ensure that all the organic matter is destroyed, as simple organic acids can interfere with the pH measurement.

Allow to cool to room temperature and measure pH.

NOTE: I. It is necessary to use analytical grade hydrogen peroxide, as commercial brands have hydrochloric acid added as a stabiliser.

2. The presence of sulphate in the oxidised solution can be confirmed qualitatively by the precipitate formed when 1 ml of 10% barium chloride solution is added to the filtered or centrifuged supernatant.

REFERENCES BATES, R. G. 1964: 'Determination of pH; Theory and Practice'. Wiley, New York.

435 p.

BLACK, C. A. 1968: 'Soil-Plant Relationships'. 2nd ed. Wiley, New York. 792 p.

FIELD ES, M.; PERROTT, K. W. 1966: The nature of allophane in soils. Part 3. Rapid field and laboratory test for allophane. N.Z. Journal of Science 9: 623-629.

FORD, H. W.; CALVERT, D. V. 1970: A method for estimating the acid sulphate potential of Florida soils. Soil and Crop Science Society of Florida Proceedings 30: 304-307.

INTERNATIONAL SOCIETY OF SOIL SCIENCE COMMITTEE ON SOIL REACTION MEASUREMENTS, 1930:·Part I. Results of Comparative Investigations on the Quinhydrone Electrode Method. Soil Research 2: 77-139.

PEECH, M. 1965: Hydrogen-ion activity. Agronomy 9: 914-926.

13

3: CARBON

At NZ Soil Bureau, two methods for determination of carbon in soils are in current use.

The method more often used is the induction-furnace method, in which the carbon is converted to carbon dioxide, which is measured volumetrically (Searle 1967). This method has the advantages of being rapid, convenient for most soils, and accurate. It measures total soil carbon content, which includes that of any carbonate or live root material present. In most New Zealand soils the contribution of carbon from these materials is negligible.

The alternative is a colorimetric method which has the advantage that it is more specific to humified organic carbon. This method involves a wet oxidation of the carbon by dichromate, and the measurement of the amount ofreduced chromium present. The disadvantage of this method is that direct calibration is not possible. Therefore calibration is made against amounts of sucrose which have the same reducing power on dichromate (measured colorimetrically) as have known amounts of soil carbon. The relationship between sucrose carbon and soil carbon is calculated from figures found by previous experience with Metson's (1956) adaption of the Schollenberger (1927a, b, 1931, 1945) and Allison (1935) methods, and the dry combustion method.

Methods of determining organic carbon in soils have been reviewed by Melson et al.( 1979).

The fraction of carbon extractable by pyrophosphate reagent is also determined.

3A TOTAL CARBON USING HIGHFREQUENCY INDUCTION FURNACE

The following method refers particularly to the equipment produced by the Laboratory Equipment Corporation (Leco), St Joseph, Michigan, U.S.A. The method involves the purification and measurement of carbon dioxide (CO,) evolved when a sample is heated in a stream of oxygen. The heat is produced by a high-frequency electrical flux induced in a mixture of the sample and of a conducting matrix of iron chips, and temperatures in excess of 1400°C can be attained. The carbon dioxide is purified by passing it in tum through a dust trap, through precipitated silver to remove chlorine, through a converter furnace to convert any carbon monoxide (CO) to carbon dioxide, and through manganese dioxide (MnO,) to remove sulphur dioxide (SO,) (Fig. !). The measurement of carbon dioxide is carried out by passing the remaining gases (CO, + 0 2) into a calibrated burette in which 5% H2S04 is displaced. The gases (CO, + 0,) are flushed through concentrated potassium hydroxide solution which absorbs the carbon dioxide. The difference between the original volume of gas in the burette and the volume of carbon dioxide-free gas is equal to the volume of carbon dioxide evolved from the sample. After correction for temperature and pressure, the carbon content of the sample is calculated.

14

This method measures total carbon so the result obtained will be directly affected by the presence of calcium carbonate, undecomposed wood, charcoal, etc.

The operation of the induction furnace and associated equipment is adequately described by the makers, so they are not included here. The following method concentrates on details required for soil analysis.

PREPARATION OF REAGENTS IRON CHIPS. Leco cat. no. 501-077.

TIN METAL. Leco cat. no. 501-076.

SILICIC ACID or pure silica.

PRECIPITATED SILVER METAL. Precipitate from silver nitrate (AgN03)

solution with sodium dithionite solution. Filter, wash precipitate with water, and dry.

PROCEDURE Use finely ground soil (air-dry, < 0.25 mm) and take sample weights according to the following table:

Carbon expected (%)

0.0-1.5 1.5-6

6-15 15-60

Sample weight (g)

1.00 0.25 0.10 0.025

For loading the crucible use a scoop which delivers about 1 g iron chips, and charge the crucible in the following order:

1-g sample

I scoop iron chips soil sample Y2 scoop tin metal 2 scoops iron chips

less than 1-g sample

I scoop iron chips soil sample 1 scoop silicic acid or pure silica 112 scoop tin metal 2 scoops iron chips

Prepare blank crucibles, loaded as for samples, and run these until a zero reading is obtained (usually after 2 or 3 blanks), before proceeding with samples.

Place the loaded crucible on the pedestal and adjust the rate of oxygen flow from a cylinder to 0.8 litres/minute.

Raise the pedestal into the induction field. This simultaneously closes the system and initiates induction, the combustion gases being passed through the dust and chlorine traps, the carbon monoxide converter furnace, and the sulphur trap before they reach the gasometric carbon analyser (Fig. I).

When the gases have displaced most of the H,SO, in the burette, stop the reaction and oxygen flow by lowering the pedestal.

Measure the uncorrected carbon value directly as 'percent carbon' from the appropriate scale on the calibrated burette.

02+C02 -----.t etc. from corn bustion tube

,---- To gasometric carbon analyser

1:~ lo~

Sulphur trap

Manganese dioxide

Carbon monoxide converter furnace

lo·°'"'S-+- Copper oxide chips

lo

'~~

:.:-:: ·.·: .. . . . :.::-:· -: . : .. . . . ... . . . · .. .... . ·:

Chlorine trap

Precipitated silver metal

Dust trap

Calico filter

Figure 1 Flow diagram for carbon determination using Leco induction furnace

15

16

CALCULATION OF RESULTS Multiply the 'percent carbon' by the appropriate corrections.for sample weight, temperature and barometric pressure (see following tables). ·

oc

21 22 23 24 25 26 27 28 29 30

Sample weight corrections

Sample weight (g)

1.00 0.25 0.10 0.025

Reading scale

I g 0.25 g

I g 0.25 g

Correction factor

10 10

FACTOR CHART FOR LECO CARBON FURNACE

Atmospheric pressure (mm) 740 745 750 755 760 765 770 775

0.950 0.957 0.963 0.970 0.976 0.983 0.990 0.996 0.945 0.952 0.958 0.965 0.972 0.978 0.985 0.991 0.941 0.947 0.954 0.960 0.967 0.973 0.980 0.986 0.936 0.942 0.949 0.955 0.962 0.968 0.975 0.981 0.931 0.937 0.944 0.950 0.957 0.963 0.970 0.976 0.926 0.932 0.939 0.945 0.952 0.958 0.965 0.971 0.921 0.927 0.934 0.940 0.946 0.953 0.959 0.966 0.916 0.922 0.928 0.935 0.941 0.948 0.954 0.961 0.910 0.917 0.923 0.930 0.936 0.942 0.949 0.955 0.905 0.911 0.918 0.924 0.931 0.937 0.943 0.950

780

1.003 0.998 0.993 0.988 0.983 0.978 0.972 0.967 0.962 0.956

'0 C' is temperature of gas as read from thermometer in measuring burette.

'Atmospheric Pressure (mm)' is pressure as read from a mercury or aneroid barometer. As mercury and steel expand with temperature, it is necessary to correct the mm reading for surrounding air temperature. In an aneroid barometer, this is accomplished by using an instrument with a temperature compensated movement. With a mercury barometer the correction is made simply by reference to the following chart, which brings the pressure to what it would be at 0°C:

Room Subtract from temperature uncorrected

(OC) reading

5-12 Imm 13-20 2mm 21-28 3mm 29-35 4mm 36-40 5mm

This corrected pressure is then used in the factor chart above.

Carbon is calculated as

reading x sample weight factor X temperature and pressure ~ C (%)

Use moisture factor (Method I} to correct results to oven-dry basis.

17

NOTES (1) Use ofsilicic acid or silica in mixture. One effect of the silicic acid or silica

is to increase the viscosity of the molten material, thus decreasing crucible failures caused by the permeation of the melt through the crucible walls. While there_ is usually sufficient silica in soils to wevent this cause of failure, there is not sufficient in blanks or organic samples (e.g., litters, peats), so it is advisable always to include silicic acid or silica in the crucible charge when less than 1 g of sample is used.

(2) Constant temperature. A supply of temperature-controlled water to the outer jacket of the carbon dioxide analyser is recommended, especially when the temperature of the water from the mains supply is variable or when it differs appreciably from room temperature.

(3) Cleaning of dust trap and gas delivery tube. The calico filter and delivery tube leading to the dust trap must be cleaned and dried regularly to prevent adsorption of carbon dioxide by moisture and dust.

(4) Sulphur trap. Moist manganese dioxide has a tendency to absorb carbon dioxide and cause low results. The effect of moisture on the manganese dioxide can be minimised by placing the sulphur trap immediately after the carbon monoxide converter furnace. It has been found that fresh manganese dioxide also absorbs carbon dioxide, so it is necessary to counteract this effect by passing copious amounts of carbon dioxide through the trap and then flushing with oxygen to remove any excess carbon dioxide. Experience has shown that as long as the sulphur trap is kept moisture-free, renewal of the manganese dioxide and subsequent carbon dioxide treatment are not frequently required.

(5) Use of variable transformer. Most furnaces are fitted with an automatic cutout device which operates if the safe current for the induction coil is exceeded. To prevent this happening during an analysis it has been found necessary to use a variable transformer to limit the induction current to a safe level.

18

3B : COLORIMETRIC DETERMINATION OF ORGANIC CARBON

In 1934, Walkley and Black published a colorimetric method for carbon determination. This' was modified by Walkley in 1947, and the method given in Metson (1956) is derived from the two papers.

The following method is essentially that of Metson with several minor changes.

PREPARATION OF REAGENTS CHROMIUM TRIOXIDE, (approx. 3M). 332.5 g chromium trioxide per litre. Dissolve in water, make to volume and filter through sintered glass or glass fibre filter paper before storing (the solution must be quite clear). The exact strength is unimportant

SULPHURIC ACID, 98% w/w. This should be used fresh from the bottle, and not left standing in a burette or beaker as it rapidly picks up moisture from the air.

SUCROSE, powdered. If necessary, grind, using a clean mortar and pestle, until particles are about 0.25 mm in diameter.

PREPARATION OF STANDARDS FOR SOILS EXPECTED TO CONTAIN LESS THAN 10% C. Weigh out sucrose standards which are equivalent to 0, 20, 40, 60, 80 and l 00 mg soil carbon according to the following table:

For 1-g soil samples

Sucrose carbon Soil carbon Soil carbon equivalent Sucrose

(%) (mg) (mg) (g)

0 0 0 0 2 20 J9.6 0.0465 4 40 39.0 0.0926 6 60 58.4 O.J386 8 80 77.9 O. J849

JO JOO 97.4 0.23J2

FOR SOILS EXPECTED TO CONTAIN MORE THAN 10% C. Weigh out sucrose standards which are equivalent to 0, 20, 40, 60, 80 and 100 mg soil carbon according to the following table:

For 0.2-g soil samples

Sucrose carbon Soil carbon Soil carbon equivalent Sucrose

(%) (mg) (mg) (g)

0 0 0 0 10 20 20.8 0.0494 20 40 41.6 0.0988 30 60 62.4 O.J48J 40 80 83.2 O.J974 50 JOO J04.0 0.2469

19

PROCEDURE FOR SOILS EXPECTED TO CONTAIN LESS THAN 10% C. Weigh out 1.00 g finely ground soil (air-dry, < 0.25 mm ) into a 200-ml Taylor-type flask which is calibrated to show the 200-ml level.

FOR SOILS EXPECTED TO CONTAIN MORE THAN 10% C. Weigh out 0.200 g finely ground soil (air-dry, < 0.25 mm ) into a 200-ml Taylor-type flask calibrated to show the 200-ml level.

To all soil samples and sucrose standards, add i2 ml cone. H,S04 (from a 250-ml rapid-flow dispensing burette) and swirl gently.

Stand for 10 minutes with occasional swirling.

Add 6 ml 3 M chromium trioxide solution (from a rapid automatic pipette) and mix well by swirling, ensuring that no particles are left on the sides of the flask.

Stand for exactly 10 minutes and then dilute with water nearly to the 200-ml mark. It has been found that the addition of chromium trioxide and the subsequent dilution can be conveniently carried out at 30-second intervals.

Let the diluted solution stand overnight, preferably at a room temperature of about 20'C.

After 18-24 hours, make up to the 200-ml mark with water and stir well.

Centrifuge an aliquot for 10-15 minutes at 2000 rpm.

Read adsorption at 600 nm on a spectrophotometer.

CALCULATION OF RESULTS Prepare a standard curve from the absorbance readings for the standards and read off concentrations of samples as mg C.

Calculate the % carbon as follows

For I g sample

For 0.2 g sample

(mg) ~ C in soil (%) 10

(mg) . . -- ~ C m soil (%)

2

Use moisture factor (Method 1) to correct to oven-dry basis.

20

3C PYROPHOSPHATE-EXTRACTABLE CARBON (Cp) This reagent extracts low molecular weight organic matter associated with iron and aluminium in complexes and short-order minerals such as ferrihydrite. Soil Taxonomy requires Cp values to identify spodic horizons when pyrophosphateextractable. Fe (Fep) values are less than 0.1 %.

PREPARATION OF REAGENTS Extracting Reagent See method 8C.

PROCEDURE Prepare extract as described in Method 8C.

Pipette 10 ml of extract into 30-ml teflon beakers and evaporate on a sandbath to low volume. Do not allow to go dry.

Add 1 scoop of silica sand and 1 scoop Fe chips to a Leco crucible and place on sandbath at least 10 minutes before they are needed.

Carefully and in stages transfer the evaporated extract to the crucible. Take care that the liquid does not soak through the crucible.

Wash any residue of extract in the beaker into the crucible using a minimal amount of water, and .leave crucible on sandbath until completely dry.

Add 'h scoop tin metal and 2 scoops Fe chips and determine carbon content on Leco furnace (use l-g scale) as described in Method 3A.

CALCULATION OF RESULTS Using a l:IOO soil:solution ratio, and a 10-ml aliquot

reading (l-g scale) X temperature and pressure correction X 10 ~ pyrophosphate-extractable carbon (Cp) %.

Use Moisture Factor (Method l) to correct to oven-dry basis.

REFERENCES ALLISON, L. E. 1935: Organic soil carbon by reduction of chromic acid. Soil Science 40:

311-320

METSON, A. J. 1956: Methods of chemical analysis for Soil Survey Samples. N.Z. Soil Bureau Bulletin I 2: 208 p.

METSON, A. J.; BLAKEMORE, L. C.; RHOADES, D. A. 1979: Methods for the determination of soil organic carbon : a review, and application to New Zealand soils. N.Z. Journal qf Science 22: 205-228.

SCHOLLENBERGER, C. J. l 927a: A rapid approximate method for determining soil organic matter. Soil Science 24: 65-68.

SCHOLLENBERGER, C. J. l 927b: Exchangeable hydrogen and soil reaction. Science 65: 552-553.

SCHOLLENBERGER, C. J. 193 l: Determination of soil organic matter. Soil Science 31: 483-486.

SCHOLLENBERGER, C. J. 1945: Determination of soil organic matter. Soil Science 59: 53-56.

SEARLE, P. L. 1967: Determination of carbon, calcium carbonate and sulphur in soil using high-frequency induction furnace equipment. N.Z. Soil News 5: 168-180.

WALKLEY, A. 1947: A critical examination ofa rapid method for determining organic carbon in soils-effect of variations in digestion conditions and of inorganic soil constituents. Soil Science 63: 251-264. -

WALKLEY. A.; BLACK, I. A. 1934: An examination of the Degtjareff method for detern1ining soil organic matter, and a proposed modification of the chromic acid titration method. Soil Science 37: 29-38.

21

4: NITROGEN

Method 4A is a semi-micro method which employs a digestion to convert nitrogen present in the sample to ammonium sulphate. Ammonium-nitrogen is subsequently determined either by distillation and titration or by a colorimetric AutoAnalyzer method.

For samples likely to have greater than 20 µg NO,/g (e.g., litters, composts, plant materials, bioturbic soils), Method 4B should be used, as it incorporates a modified digestion mixture to reduce nitrates to ammonium sulphate.

4A TOTAL NITROGEN WHERE NITRATE IS PRESENT IN TRACE AMOUNTS ( < 20 µg N03/g) The method described is a modified semi-micro Kjeldahl method in which the digestion is carried out in 50-ml calibrated test tubes inserted in a drilled aluminium block on a hotplate. Two alternative determination steps are described: I, the traditional steam distillation titration procedure and II, a colorimetric AutoAnalyzer method described by Searle (1975).

The colorimetric determination is based on the reaction of ammonia with a phenol (sodium salicylate) in alkaline oxidising conditions to form an indophenol which absorbs strongly at 660 nm. The reaction is catalysed by sodium nitroprusside, and a citrate-tartrate mixture is employed to chelate metals that would otherwise form insoluble hydroxides. A review of the reaction and its analytical uses is given by Searle (1984).

APPARATUS DIGESTION TUBES. 'Exelo' 50-ml calibrated test tubes cat. no. T2/30.

ALUMINIUM HEATING BLOCK. Aluminium block (30 cm X 30 cm X 5 cm) drilled with seven rows of six holes (2. 7 cm diameter, 3.5 cm depth).

HOT PLATE. Domestic single radiant element hotplate (element should be at least 20 cm diameter and have an output of about 2 kW).

PREPARATION OF REAGENTS SULPHURIC ACID, CONC.

KJELDAHL CATALYST TABLETS. B.D.H. Cat. No. 33064. Each tablet contains 1 g sodium sulphate and 0.1 g copper sulphate.

PROCEDURE Weigh 0.500 g finely ground (air-dry, < 0.25 mm) soil into a dry 50-ml calibrated test tube. For peaty soils use a smaller sample-0. l 0 to 0.20 g-to avoid excessive frothing.

A reagent blank should be carried throughout the following procedure.

22

Moisten the soil with a few drops of water and allow the moisture to penetrate the soil. Lower results have been reported for nitrogen determinations on certain heavy clay soils where this slight wetting was omitted (Bal 1925; Walkley 1935; Alper 1938).

Add a Kjeldahl catalyst tablet and 3.25 ml of cone. H2S04 • Significant errors will occur in the colorimetric AutoAnalyzer method (Method 4A.II) if the final acid content of the digest falls outside the range 2.75-3.25 ml of cone. H,S04 • Some soils, particularly calcareous soils, have been found to neutralise up to 0.5 ml cone.' H2S04 per 0.5 g sample.

Place the tubes in a preheated aluminium digestion block on a radiant-element hotplate.

Boil the digestion mixture until it decolorises (usually 20-30 min.). Soils high in organic matter need careful watching as frothing occurs. The digestion is then carried on for another 20-30 min. to ensure conversion of all nitrogen to ammonium sulphate.

Remove tubes from block.

Before the digests are completely cold (5-6 min. after removal), add carefully 10-15 ml of distilled water and swirl to hasten the solution of salts.

When cool make to 50-ml mark with distilled water, stopper, and shake vigorously. Allow solids to settle (usually overnight).

The entire digest may be used for the distillation method (Method 4A.I) by not diluting to 50 ml. However, it is necessary to add about JO ml of water to the digest to prevent it solidifying.

4A.I MANUAL METHOD

PREPARATION OF REAGENTS BORIC ACID, 1 % SOLUTION.

HYDROCHLORIC ACID, 0.02 M.

BROMOCRESOL GREEN-METHYL RED MIXED INDICATOR (Ma and Zuazaga 1942). Mix 5 parts 0.1% bromocresol green in 95% ethanol with 1 part 0.1 % methyl red in 95% ethanol. These proportions may have to be varied to obtain the neutral grey transition colour of the indicator.

SODIUM HYDROXIDE, 1 + 2. Dissolve I kg NaOH pellets in 2 litres water, stirring constantly.

PROCEDURE Transfer total sample digest or aliquot (usually 20 ml) to distillation apparatus.

Add approximately 10 ml 1 + 2 NaOH. A little pH indicator will show whether sufficient Na OH 'has been added, but usually the formation of a brown precipitate of ferric hydroxide, when the liquids are mixed, indicates neutralisation of the acid.

Rinse the funnel with a little water.

Add 8-10 ml I% boric acid* and 5-6 drops of indicator to a 100-ml Erlenmeyer flask and place under the delivery tube of the condenser so that the tip is below the surface of the liquid.

*At least 5000 µg of N can be fixed by 10 ml 1% boric acid, with Jess than 0.5% error (Yuen and Pollard 1953). The more dilute the boric acid that can be used with complete retention of ammonia the sharper the end-point of the titration. '

23

Close sample inlet and drainage outlet and pass steam into the distillation flask. The liquid will soon boil, and the indicator in the boric acid solution will change colour as soon as ammonia begins to distil over.

After a minute or two, lower the flask so that the tip of the delivery tube is clear of the liquid. No. ammonia will be lost if the condenser is efficient. The temperature of the distillate should not rise above 40°C (Yuen and Pollard 1953).

When about 20-25 ml of distillate have collected, rinse the tip of the tube with a little water and remove the flask.

Stop the entry of steam. The distilling flask will empty automatically and the vacuum can be used to rinse the apparatus by immersing the tip of the delivery tube in water.

Titrate the distillate against 0.02 M HCI, to the neutral grey colour of the indicator.

Correct titration for reagent blank.

CALCULATION OF RESULTS For total digests

0.5 g soil: 0.02 M HCI (ml) X 0.056 ~ N (%)

0.2 g soil: 0.02 M HCI (ml) X 0.140 ~ N (%)

For aliquot (from 50 ml volume)

50 0.5 g soil: 0.02 M HCI (ml) X 0.056 X

1. ( I) ~ N (%)

a 1quot m

50 0.2 g soil: 0.02 M HCI (ml) X 0.140 X

1. ( l) ~ N (%)

a 1quot m


4A.II. AUTOANALYZER

PREPARATION OF REAGENTS DICHLORO-S-TRIAZINE-2,4,6 TRIONE, SODIUM SALT (NaDTT), (Koch Light Cat. No. 1615 h). Dissolve 10 g NaOH in 400 ml water, add 0.25 g NaDTT and make to 500 ml.

SODIUM SALICYLATE. Dissolve 15 g sodium salicylate in water and make to 1 litre.

SODIUM NITROPRUSSIDE. Dissolve 1.2 g sodium nitroprusside in water and make to 500 ml.

CITRATE-TARTRATE REAGENT. Dissolve 25 g NaOH in 800 ml water, add 6 g tri-sodium citrate and 18 g sodium hydrogen tartrate, dissolve and make to 1 litre with water.

WASH SOLUTION. Carefully add 120 ml cone. H2S04 to 1500 ml water, add 40 g Na2S04 and 4 g CuS04.5H20, dissolve and make to 2 litres with water.

PREPARATION OF STANDARDS STOCK SOLUTION (500 µg N/ml). Dissolve 2.3585 g ammonium sulphate (M.A.R. or Aristar quality dried at l I0°C) in approximately 800 ml water. Carefully add 60 ml cone. H,SO,, 20 g of Na,SO,, 2 g CuS0,.5H20, dissolve and make to 1 litre with water in a volumetric ilask.

WORKING STANDARDS. Pipette 2.5; 5, 10, 15, 20, 25, 40, and 50 ml of stock solution into 250-ml volumetric flasks, and make each to volume with wash

24

3 ml heating bath coil

45°C

colorime1er 660 nm

15 mm flow cell

with wash solution (see preparation of reagents). These standards contain 5, 10, 20, 30, 40, 50, 80, and 100 µg N/ml respectively.

PROCEDURE Set up AutoAnalyzer as shown in flow diagram (Fig. 2) with heating bath at 45°C and 660 nm filters in the colorimeter.

Pump reagents and wash solution through system for about 10 min. to ensure complete flushing of the analytical system and wash receptacle.

Pour standard and sample solutions into cups on the sample tray, set up the recorder baseline with the colorimeter baseline control.

Sampl~ top standard and when it reaches the colorimeter flow cell set recorder to read 100% absorption, using the standard calibration control. on colorimeter (a standard calibration of about 1 is usual). Reset baseline if necessary.

Sample standards and unknowns at a rate of 60/hr with a 5: I sample:wash ratio.

Digests containing more than 50 µg N/ml can be diluted using the wash solution as diluent. Soils with extremely high nitrogen contents should be redigested using a smaller weight of soil.

CALCULATION OF RESULTS Prepare a standard curve of µg N/ml against absorbance.

For 0. 5 g of soil made to 50 ml

µg N/ml 100 ~ N (%)

Apply a blank correction, then use moisture factor (Method 1) to correct to ovendry basis.

Flow Rare Reagent

ml/min

5 turn 0.32 air mixing coil

0.8 corn plexing agent

0.1 sample

0.6 nadtt

0.23 nitroprusside

1.2 salicyla1e

1.0 wash

waste 2.0 pull from colorime1er

Figure 2 Flow diagram of AutoAnalyzer manifold for total nitrogen and CEC determinations

Tube Colour Code

black

red

orange/green

white

orange/white

yellow

grey

green

4B: TOTAL NITROGEN WHERE NITRATE CONTENT IS HIGH ( > 20µg N03/g)

25

NOTE: The digestion used in this method is also suitable for the determination of total potassium and phosphorus in plant material.

PREPARATION OF REAGENTS KJELDAHL CATALYST TABLETS. See Method 4A.

SULPHURIC ACID-SALICYLIC ACID MIXTURE. Dissolve 20 g salicylic acid in 600 ml cone. H2S04•

SODIUM THIOSULPHATE, fine crystals.

PROCEDURE Weigh a 0. 100-g sample into a 50-ml calibrated tube (plant material needs to be dried for 2 hours at 105'C before weighing). ·

Add 4.0 ml sulphuric acid-salicyclic acid mixture and allow to stand with occasional shaking for l hour. Stopper tubes and leave overnight.

Add 0.25 g sodium thiosulphate through a long-necked funnel and heat gently for about 5 min.

Remove from heat, and when cool, add a Kjeldahl catalyst tablet and carry out the digestion as described in Method 4A.

A reagent blank should be carried throughout the procedure.

4B.I MANUAL METHOD Carry out digestion, distillation, and titration according to Method 4A, but use approximately 20 ml l + 2 NaOH for the distillation.


CALCULATION OF RESULTS For total digest

O. l g sample: 0.02 M HCl (ml) X 0.280 ~ N (%)

For aliquot 50

O. l g sample: 0.02 M HCl (ml) x 0.280 x . ~ N (%) aliquot

Use moisture factor (Method l) to correct to oven-dry basis.

4B.II : AUTOANALYZER

PREPARATION OF REAGENTS See Method 4A.ll.

PREPARATION OF STANDARDS STOCK SOLUTION (500 µg N/ml, 500 µg K/ml and 50 µg1P/ml). Dissolve 2.3585 g ammonium sulphate, (NH,),S04 (M.A.R. or Aristar quality, dried at l lO'C), 0.9533 g potassium chloride, KC! (dried at I JO'C), and 0.2292 g disodium hydrogen orthophosphate, Na2HPO, (dried at I IO'C), in water. Add 0.5 ml toluene as a preservative and make to l litre in a volumetric flask.

26

WORKING STANDARDS. Prepare 6 blank digests as described above but add approximately 20 ml water instead of making to 50 ml.

Pipette 0, 1, 2.5, 5, 7.5, and 10 ml of stock solution into the blank digests. Make to 50 ml with water.

These standards contain 0, 10, 25, 50, 75 and JOO µg N and K/ml and 0, 1.0, 2.5, 5.0, 7.5, and 10.0 µg P/ml.

PROCEDURE As for Method 4A.II ·

CALCULATION OF RESULTS Prepare a standard curve of µg N/ml against absorbance.

For 0.1 g sample made to 50 ml

µg N/ml 20

= N (%)

NOTE: Determination of potassium in digest.

Compare potassium concentration in samples with standards, using flame emission spectrometry at 766.5 nm.

µg ~6ml = K (%)

Determination of phosphorus in digest.

Prepare I + I dilutions of samples and standards with water and determine Pas for SA.II. The dilution is necessary to make the solution 0.5 M with respect to H 2so ..

µg ~bml = P (%)

Apply a blank correction, then use moisture factor (Method I) to correct to ovendry basis.

27

4C 2 M KCI-EXTRACTABLE NITRATE

KCl-extractable nitrate is useful in nitrogen balance and leaching studies and is valuable for assessing plant-available nitrate. Aerobic incubation tests, which are among the best predictors of soil nitrogen availability, also require the measurement of extractable nitrate.

Nitrate is determined by an AutoAnalyzer method which is adapted from that ofKamphake et al. (1967). Hydrazine sulphate is used to reduce nitrate to nitrite which is then determined colorimetrically, using a diazotization coupling reaction. The method can also be used for determining nitrite by omitting the reduction step.

PREPARATION OF REAGENTS 2 M POTASSIUM CHLORIDE. Dissolve 1490 g KC! in water and make to 10 litres.

COLOUR REAGENT. Carefully add 40 ml phosphoric acid, H,P04, to 300 ml water. Make a suspension of 16 g sulphanilimide plus 0.8 g N-1-naphthylethylenediamine dihydrochloride (NEDD) in 50 ml water. Add this suspension to the first solution, stirring vigorously to dissolve. Make up to 400 ml and filter if necessary. Store in the dark and make up fresh solution after 2-3 days.

STOCK COPPER SOLUTION. Dissolve 3.9 g copper sulphate, CuS0.-5H,O, in 1 litre of water.

WORKING COPPER SOLUTION. Dilute 4 ml of stock copper solution to 500 ml with water. Add about 5 drops Brij 35 wetting agent. Make up each day.

HYDRAZINE SULPHATE SOLUTION. Dissolve 0.61 g hydrazine sulphate in I litre water.

SODIUM HYDROXIDE/PYROPHOSPHATE SOLUTION. Dissolve 0.5 g tetra-sodium pyrophosphate, Na4P20,.IOH20, in 500 ml water. Add 20 g sodium hydroxide, NaOH, and dissolve.

WASH SOLUTION. Use water with the addition of2-3 drops ofBrij 35 /litre.

PREPARATION OF STANDARDS STOCK NITRATE STANDARD (100 µg NO,-N/ml). Weigh 0.7218 g potassium nitrate, KNO,, dried at 105°C, dissolve and make to 1 litre in a volumetric flask with 2 M KC!.

STOCK NITRITE STANDARD (!OOµg N02-N/ml). Weigh 0.4926 g sodium nitrite, NaNO,, dried at 105°C, dissolve and make to I litre in a volumetric flask with 2 M KC!.

WORKING NITRATE STANDARDS. Pipette 0, 2, 4, 6, 8, and 10 ml stock nitrate standard into 100-ml flasks and make to volume with 2 M KC!. These standards contain 0, 2, 4, 6, 8, and 10 µg N03-N/ml respectively. Solutions are stable for about 6 months.

WORKING NITRITE STANDARDS. As for nitrate standards above, but use the stock nitrite standard.

PROCEDURE Weigh 10.0 g field-moist soil (or moist soil incubated for mineralisation experiments) into a 250-ml conical flask and add 100 ml 2 M KC!.

Carry a reagent blank throughout the procedure.

Shake on an orbital shaker for 1 hour.

28

20 t urn mixing coil .. .... ......

Filter 15-20 ml of suspension through a Whatman no. 40 filter paper into a wideneck McCartney bottle.

Store at 4°C if not analysing immediately. These extracts ·are stable for several weeks (Ross et al. 1979).

Set up AutoAnalyzer as shown in flow diagram (Fig. 3), with heating bath at 28-290C and 520 nm filters in the colorimeter.

Pump reagents and wash solution through system for about 30 min. to ensure thorough flushing of the system.

Pour the top nitrate standard, followed by the same concentration nitrite standard, then the rest of the nitrate standards.

Set the recorder baseline with the colorimeter baseline control.

Sample top standard and when it reaches the colorimeter flowcell set the recorder to read 90-95% absorption, using the standard calibration control on the colorimeter. Reset the baseline if necessary.

Read all the standards. The peak heights should be the same for the top nitrate and nitrite standards. This is to check that all the nitrate is reduced to nitrite. If the nitrate standard is lower, raise the waterbath temperature to allow all of the nitrate to be reduced. If the nitrite standard is lower, nitrite is being reduced as well as nitrate, and the waterbath temperature should be lowered.

Sample standards and unknowns at a rate of 60/hour (40-sec sample; 20-sec wash) or 90/hour (25 sec; 15 sec).

At the end of a run, pump a few ml of approximately 10% HCl through the copper solution line to remove precipitated copper oxide.

Flow Rate R t ml/min eagen

5 10 t urn turn mixing coil mixing coil I . .. •

" ' I

0.6 air

0.8 copper solution

0.1 sample

....... • 1.11 0.16 NaOH solution . .... 10 ml

heating coil 0.42 hydrazine

0.42 distilled water

0.42 colour reagent

Tube Colour Code

white

red

orange/green

orange/yellow

orange

orange

orange

I colorimeter 520 nm

1.6 colorimeter pull blue

0.8 wash red

Figure 3 Flow diagram of AutoAnalyzer manifold for nitrate determination

CALCULATION OF RESULTS Prepare a standard curve of µg N03-N/ml against absorbance.

Read off unknowns as µg N03-N/ml in solutions.

For a I: JO soil:solution ratio

µg N03-N/ml x JO = µg N03-N/g (soil)

29


REFERENCES ALPER, Pauline 1938: An accurate wet-combustion method for the determination of car

bon in soils. Journal of Agricultural Science 75: 173-180.

BAL, D. V. 1925: The determination of nitrogen in heavy clay soils. Journal of Agricultural Science 15: 454-459.

KAMPHAKE, L. J.; HANNAH, S. A.; COHEN, J. M. 1967: Automated analysis for nitrate by hydrazine reduction. Water Research 1: 205-216.

MA, T. S.; ZUAZAGA, G. 1942: Micro-Kjeldahl determination of nitrogen. A new indicator and an improved rapid method. Industrial and Engineering Chemistry. Analytical edition 14: 280-282.

ROSS, D. J.; BRIDGER, B. A.; CAIRNS, A.; SEARLE, P. L.; 1979: Influence of extraction and storage procedures, and soil sieving, on the mineral nitrogen content of soils from tussock grasslands. NZ Journal of Science 22:143-149.

SEARLE, P. L. 1975: Automated colorimetric determination of ammonium ions in soil extracts with 'Technicon Autoanalyzer II equipment'. NZ Journal of Agricultual Research 18: 183-187.

SEARLE, P. L. 1984: The Berthelot or indophenol reaction and its use in the analytical chemistry of nitrogen. A review. Analyst 109: 549-568.

WALKLEY, A. 1935: An examination of methods for determining organic carbon and nitrogen in soils. Journal of Agricultural Science 25: 598-609.

YUEN, S. H.; POLLARD, A. G. 1953: Determination of nitrogen in soil and plant materials: use of boric acid in the micro-Kjeldahl method. Journal of the Science of Food and Agriculture 4: 490-496.

31

5 : PHOSPHORUS

The main phosphorus fractions determined by the NZ Soil Bureau Soil Analysis Section are:

5A,B,C - AVAILABLE PHOSPHORUS. Determinations are normally carried out on topsoil samples only. The values measure different forms of readily soluble phosphorus and may be used with some limitations, as a guide to soil fertility. Three methods which are often used, Truog (5A), Olsen (5B) and Bray 2 (5C), are described.

5D - 0.5 M H2SO,-SOLUBLE PHOSPHORUS. Results from this method are used to assess the amount of inorganic phosphorus which is present but not fixed. The values obtained are useful in pedological studies as they provide an indication of the state of weathering and leaching.

5E - ORGANIC PHOSPHORUS. Results, when considered as a proportion of the total phosphorus present (particularly for B horizons), are useful in the study of soil-forming processes.

5F - TOTAL PHOSPHORUS. These values are are used in conjunction with other phosphorus fractions to elucidate the state of weathering and leaching of a soil.

5G - PHOSPHATE RETENTION. This method was originally devised and briefly described by Saunders (1965). It is an empirical measure of the ability of the soil to remove phosphorus rapidly from solution. This process is considered to be a precursor to the much slower process of phosphorus fixation, which renders phosphorus unavailable to plants. In acid soils of pH < 6.5, compounds of iron and aluminium are considered to play an increasingly important role (Saunders 1968). The method has been devised so that the concentration of phosphate used gives a high degree of differentiation between soils of high and low phosphorus-retention ability, and the pH used (4.6) is close to the point of maximum phosphate retention in many soils.

Two colorimetric methods are given for the determination of phosphorus in the extracts. I, a manual procedure and II, an AutoAnalyzer procedure. These methods are applicable for all the phosphorus values determined except phosphate retention, for which a separate method is used.

32

SA TRUOG-SOLUBLE PHOSPHORUS This method is based on that of Truog (1930), and is similar to the method which was used by the Ministry of Agriculture and Fisheries to produce soil Ptest values for New Zealand soils until 1976, when it was replaced by the Olsen method. The main disadvantage is that the reagent dissolves plant-unavailable primary phosphorus compounds from. Recent soils.

PREPARATION OF REAGENT EXTRACTING REAGENT (0.001 M H2SO, buffered to pH 3). Dissolve lS g ammonium sulphate, (NH4) 2S04, in 10 ml O.S M H2SO, and dilute to S litres.

PROCEDURE The amount of phosphorus extracted depends on the extraction time which is relatively short (30 min.). Therefore it is necessary to arrange the work so that shaking time is kept as close to the 30 min. as possible, with minimum time spent in adding the extracting reagent to the soils, placing flasks on shaker, and filtering extracts after shaking.

Place O.S g soil (air-dry, < 2 mm) into a shaking bottle.

Add 100 ml extracting reagent and shake for 30 min. on an end-over-end shaker (about SO r.p.m).

Filter as soon as possible through no. 1 (Whatman) paper until at least 7S ml have been collected, if using Method SA.I. If using SA.II, filter about 18 ml into a sampler test tube. (Fill to about 1 cm from top).

A reagent blank should be carried throughout the determination.

SA.I MANUAL METHOD This colorimetric procedure is based on the method of Murphy and Riley (1962) as adapted by Watanabe and Olsen (196S). The procedure involves reduction of the phospho-molybdate complex by ascorbic acid in a reaction catalysed by antimony.

PREPARATION OF REAGENTS MURPHY AND RILEY REAGENT A. 1.2% solution of ammonium molybdate, (NH,)6Mo70 24.4H20, with 0.1 mg/ml antimony in 2.S M H2S04•

To prepare S litres of solution:

Dissolve 60 g ammonium molybdate in 1 litre water. The rate of solution may be increased by warming, but do not warm above 60°C. Cool the solution.

Dissolve 1.3343 g antimony potassium tartrate in 2SO ml water. Add both of the dissolved reagents to 2.S litres of S M H,SO, (70S ml cone. H,SO, made to 2.S litres with water). Mix thoroughly, make to S litres and store in dark bottles.

MURPHY AND RILEY REAGENT B. In each 100 ml of reagent A dissolve 1.0S6 g ascorbic acid and mix. This reagent must be prepared as required as it does not keep for more than 24 hours.

PREPARATION OF STANDARDS STOCK SOLUTION (100 µg P/ml). Dissolve 0.220 g potassium dihydrogen phosphate (KH,PO,), A.R., in water, add 0.S ml toluene as a preservative, and make up to SOO ml in a volumetric flask.

WORKING STOCK (1 µg P/ml). Pipette out S ml stock solution and dilute to SOO ml in a volumetric flask.

33

WORKING STANDARDS. Pipette 0, 5, 10, 15, 20 and 25 ml of the 1 µg P/ml solution into 100-ml volumetric flasks and dilute to approximately 80 ml with water. These standards contain 0, 5, 10, 15, 20 and 25 µg P respectively. Treat these standards as for samples.

PROCEDURE Pipette out a 50-ml aliquot of the Truog extract into a 100-ml volumetric flask and dilute to approximately 80 ml with water.

To standards and unknowns add 8 ml of Reagent B, make to 100 ml, and mix well. The colour produced is stable for 24 hours and maximum intensity is reached in 10 min.

Maximum absorption occurs at 880-885 nm. However, a secondary peak at 660 nm may be used.

CALCULATION OF RESULTS Construct a standard curve of µg PI 100 ml against absorbance. This should be near linear.

Using a 1:200 soil:solution ratio:

µg P / 100 ml X 4 ~ Truog-soluble P (µg/g)


SA.II AUTOANALYZER The AutoAnalyzer method is a modification of that proposed by Colwell (1965), who utilised the ascorbic acid reduction of the phosphomolybdate complex.

PREPARATION OF REAGENTS ACID AMMONIUM MOLYBDATE. 0.5% ammonium molybdate, (NH,)6Mo,024.4H20, in I M H,SO.- Dissolve 5 g ammonium molybdate in 800 ml water, carefully add 56 ml cone. H,SO, and cool. Add about 2 ml wetting agent (Wetting Agent A) when cool and make up to I litre.

ASCORBIC ACID. Dissolve 2.25 g ascorbic acid in 250 ml water. If kept in a refrigerator, the solution will be useable for about a week, after which fresh solution should be made up.

w ASH SOLUTION. 0.5 M H,so ..

PREPARATION OF STANDARDS STOCK SOLUTION (100 µg P/ml). Dissolve 0.220 g potassium dihydrogen phosphate, KH,PO,, in water, add 14 ml cone. H2SO, and make to 500 ml in a volumetric flask.

WORKING STANDARDS. Pipette 1- and 2-ml aliquots of stock solution (100 µg P/ml) into 200-ml volumetric flasks and make to volume with 0.5 M H,so .. These standards contain 0.5 and 1.0 µg P/ml.

PROCEDURE To 18 ml of filtered extract add 0.5 ml of cone. H,SO, with a dispenser, stopper, and shake.

Treat I 8 ml of extracting solution in the same manner to use as a blank. Cone. H2S04 is added to make the final solution 0,5 M H2S04, and this enables the Truog solutions to be analysed with the same manifold and reagents as for the other phosphate measurements. The error introduced by the addition of the small volume of acid is insignificant.

34

Set up an AutoAnalyzer as shown in manifold diagram (Fig. 4), with heating bath at 90°C and 880 nm filters in the colorimeter.

Pump reagents and wash solution for about 10 min. to ensure complete flushing of the analytical system and wash receptacle.

Pour standard and sample solutions into tubes on the sample tray.

Set recorder baseline with baseline control on colorimeter.

Sample top standard and when it reaches the colorimeter flowcell adjust recorder to read 100% transmission with the standard calibration control on the colorimeter. Reset baseline if necessary, and repeat this process to check calibration.

Sample standards and then unknowns, at a rate of 60/hr with a 5: 1 sample:wash ratio.

CALCULATION OF RESULTS Prepare a standard curve of µg P /ml against absorbance.

Using a 1:200 soil:solution ratio

µg P /ml X 200 ~ Truog-soluble P (µg/g)


Flow Rate Reagent

ml/min

5 turn 0.23 air

3 ml heating mixing coil

0.6 molybdate bath coil 90°C 0.23 sample

colorimeter 0.6 ascorbic

880 nm 15 mm flow

cell

0.8 wash

waste 1.0 pull from colorimeter

Figure 4 Flow diagram of AutoAna/yzer 1nanifold for phosphorus detennination

Tube Colour Code

orange/whije

whoo

orange/whoo

whoo

red

grey

35

SB OLSEN-SOLUBLE PHOSPHORUS This method is based on that of Olsen et al. (19S4), and is similar to that used by the Ministry of Agriculture and Fisheries to produce soil P-test values for New Zealand soils (Cornforth 1980).

PREPARATION OF REAGENTS EXTRACTING REAGENT (O.S M NaHC03). Dissolve 42.0 g sodium hydrogen carbonate in water and dilute to about 980 ml. Adjust pH to 8.S by adding approximately SO% sodium hydroxide drop by drop. Add 1 ml 0.2% superfloc solution and make to 1 litre. To prevent pH changes in the reagent, make up fresh and adjust pH immediately before use (Cowling et al. 1986).

PREPARATION OF STANDARDS STOCK SOLUTION (100 µg P/ml). See Method SA.I.

WORKING STANDARDS. Use l, 2 and 3 µg P/ml standards as described in Method SD.IL

PROCEDURE Place 1.0 g soil (air-dry, < 2 mm) into a SO-ml centrifuge tube.

Add 20 ml extracting reagent and shake for 30 min. in an end-over-end shaker (about SO r.p.m.).


Filter through no. 42 (Whatman) paper and collect 18 ml (standard test-tube full).

Either, measure and calculate P concentrations manually, using Method SA.I but adjusting the pH of the samples as described under method SD.I.

Or, pour into a SO-ml erlenmeyer flask and add 1.S ml cone. H,so .. When effervescence has ceased, pour back into 18-ml test tube and run samples and standards on the AutoAnalyzer as described in Method SA.II.

CALCULATION OF RESULTS (for AutoAnalyzer method) Prepare a standard curve of µg P /ml against absorbance

Using a 1 :20 soil:solution ratio

µg P/ml (in extract) X 20 X 1.08 ~ Olsen P (µg/g)

NOTE. The 1.08 factor is to correct for dilution introduced by adding 1.S ml cone. H,so ..


36

SC BRAY 2-SOLUBLE PHOSPHORUS

This method is based on that of Bray and Kurtz (l 94S), and has been found to give a good correlation between growth of pine tree seedlings and P level for New Zealand soils (Ballard 1974).

PREPARATION OF REAGENTS 1 M AMMONIUM FLUORIDE. Dissolve 18.S2 g NH,F in water and make to SOO rril with water.

O.S M HCI. 43 ml cone. HCl (sp.gr. 1.18) made to 1 litre.

EXTRACTING REAGENT (0.3 M NH4F + 0.1 M HCI). Add 60 ml 1 M NH,F and 400 ml O.S M HCI to a 2-litre flask, and make to volume.

PREPARATION OF STANDARDS STOCK SOLUTION (100 µg P/ml). See Method SA.I.

WORKING STANDARDS. Pipette 0, 1, 2.S, S, 7.S and 10 ml 100 µg P/ml stock solution into 100-ml volumetric flasks. Add 3 ml 1 M NH,F, 20 ml O.S M HCI, and 2.8 ml H2SO .. Make to volume with water.

These standards contain 0, 1.0, 2.S, S.0, 7.S and 10.0 µg P/ml.

PROCEDURE Place 2.S g soil (air-dry, < 2 mm) into a SO-ml centrifuge tube.

Add 2S ml extracting reagent and shake for 40 seconds.


Filter immediately through no. 30 (Whatman) paper and collect 18 ml (standard test-tube full).

Either, measure P concentrations manually, using Method SA.I but adjusting the pH of the samples as described under method SD.L

Or, add O.S ml cone. H2S04 and run samples and standards on the AutoAnalyzer as described in Method SA.II. The acid is added to make the solutions 0.S M

with respect to H2S04 so that the same reagents as used for method SA.II may be used. The dilution error introduced by the 0.5 ml of acid added is not considered significant, but may be corrected for if desired.


Using a 1:10 soil:solution ratio

µg P/ml (in extract) X 10 ~ Bray 2-soluble P (µg/g)

Apply a blank correction, then use moisture factor (Method l) to correct to ovendry basis.

37

5D 0.5 M H2S04-SOLUBLE PHOSPHORUS

PREPARATION OF REAGENTS 0.5 M H,so .. Ma.ke up 280 ml cone. H,SO, to 10 litres (add the acid carefully, then cool). Check molarity by titrating an aliquot with standard alkali.

PROCEDURE Place 0.500 g of finely ground soil (air-dry, < 0.25 mm) into a 250-ml shaking jar and add l 00 ml 0.5 M H,so ..

Shake on an end-over-end shaker (about 50 r.p.m.) for 16 hr (overnight) under temperature-controlled conditions (20°C).

Filter through a no. 42 (Whatman) 12.5 cm filter paper and collect 40-50 ml clear extract for method 5D.I (Manual).

If using method 5D.II (automated), collect about 18 ml in a sampler tube.

Include a reagent blank.

SD.I : MANUAL METHOD

PREPARATION OF REAGENTS p-NITROPHENOL INDICATOR. Dissolve 0.5 g p-nitrophenol in 25 ml water.

AMMONIUM HYDROXIDE, 1 + I. One part of cone. NH,OH (sp.gr. 0.88-0.90) plus one part water.

MURPHY AND RILEY REAGENTS A and B. See Method 5A.I.

PREPARATION OF STANDARDS WORKING STOCK SOLUTION (1 µg P/ml). See Method 5A.I.

WORKING STANDARDS. Prepare standards by pipetting 0, 5, 10, 15, 20, and 25-ml aliquots of working stock phosphate solution (I µg P/ml) into 100-ml volumetric mixing flasks and make to about 80 ml with water. These solutions contain 0, 5, 10, 15, 20 and 25 µg P respectively. Treat these standards as for samples.

PROCEDURE Pipette an aliquot of the filtrate containing not more than 25 µg P into a 100-ml volumetric (mixing) flask; a 10-ml aliquot will be suitable for most soils.

Dilute with water to about 80 ml and then adjust the pH to about 4.5 to 5.5 as follows: add 1 drop of p-nitrophenol indicator and then add 1 + 1 NH,OH drop by drop until the solution just turns yellow. For a 10-ml aliquot, this should take from 1.5 to 2 ml (depending on the strength of the NH,OH).

Add 0.5 M H2S04 drop by drop until the solution just loses its yellow colour. When organic matter causes a yellow tinge in the solution, it is advisable to add 2 drops of p-nitrophenol indicator and to make the pH adjustment according to the change in indicator colour only.

Some soils give a precipitate of iron and aluminium hydroxides, but this can be disregarded, as it redissolves when the acid molybdate is added during colour development.

Develop and measure the molybdenum blue colour, using Murphy and Riley molybdate reagent according to the procedure described under Method 5A.I.

38

CALCULATION OF RESULTS Prepare a standard curve of µg P / 100 ml against absorbance. This should be near linear.

For a 1:200 soil:solution ratio

(µg P in final 100 ml) X 20 ~ 0.5 M H2S04-soluble P (mg/100 g)

aliquot (ml)


SD.11: AUTOANALYZER

PREPARATION OF REAGENTS ACID AMMONIUM MOLYBDATE. As for Method SA.II.

ASCORBIC ACID. As for Method SA.II.

WASH SOLUTION. 0.S M H2SO, extraction solution.

PREPARATION OF STANDARDS STOCK SOLUTION (JOO µg P/ml). See Method SA.II.

WORKING STANDARDS. Pipette 2, 4, 6, 8, 10, 12, 16, and 20-ml aliquots of stock solution (100 µg P/ml) into 200-ml volumetric flasks and make to 200 ml with O.S M H,S04• These standards then contain 1, 2, 3, 4, S, 6, 8, and 10 µg P/ml.

PROCEDURE Set up the AutoAnalyzer as described in Method SA.II.

Sample unknowns at a rate of 60/hr with a S: 1 sample:wash ratio, using the appropriate range of standards (1-S µg P/ml or 2-10 µg P/ml).


For a 1 :200 soil:solution ratio

µg P/ml X 20 ~ 0.S M H,SO,-soluble P (mg/100 g)


39

SE ORGANIC PHOSPHORUS

This phosphorus fraction is determined from the increase in 0.5 M H,S04-soluble phosphorus caused by ignition of the soil, which converts organic phosphorus to inorganic pho~phate. It has been found that in some soils with very low base status, phosphorus may be lost during ignition. To prevent this, calcium acetate is added prior to ignition.

In strongly weathered soils, ignition causes positive errors due to solubilisation of some forms of inorganic phosphorus (which were previously insoluble in 0.5 M

H2S04). In some tropical soils these errors are sufficient to render the results for organic phosphorus invalid.

PREPARATION OF REAGENTS CALCIUM ACETATE, 20%. Dissolve 100 g anhydrous salt, by heating in 400 ml water. Add about 5 ml glacial acetic acid to aid solution and make up to 500 ml with water.

0.5 M H,so .. See Method 5D.

PROCEDURE Weigh 0.500 g finely ground soil (air-dry, < 0.25 mm) into a 5-ml silica crucible.

Wet with 0.5 ml calcium acetate solution (20%) and dry in an oven (1 lO°C).

Ignite for 60 min. at 550°C, cool, and drop crucible into a 250-ml shaking jar containing 100 ml 0.5 M H,so ..

Carry a reagent blank throughout the procedure.

Carry out extractions as for Method 5D.

SE.I MANUAL METHOD Carry out colour development and calculation as described in Method 5D.l. This calculation gives the amount of ignited 0.5 M H2S04 soluble phosphorus.

SE.II : AUTOANAL YZER Carry out determination and calculation described in Method 5D.Il. This calculation gives the amount of ignited 0.5 M H2SO, soluble phosphorus.

CALCULATION OF RESULTS

Ignited 0.5 M H2S04-sol. P (mg/100 g) - 0.5 M H2S04-sol. P (mg/100 g) ~ Organic P (mg/100 g)

40

SF TOTAL PHOSPHORUS Two fusion methods are described. The first (5F.1) is a NaHC03 fusion using platinum crucibles (Syers et al. 1968) and the second (5F.2) uses NaOH fusion and nickel crucibles (Smith and Bain 1982). The latter method has the advantages that expensive platinum crucibles are not required, and that the fusion is easier to perform. ·

SFl FUSION WITH NaHC03

PREPARATION OF REAGENTS SULPHURIC ACID, 1 + 3. Carefully add 250 ml cone. H,SO, to 750 ml water. Cool in running water.

SODIUM CARBONATE, anhydrous Na2CO,.

PROCEDURE Weigh 0.25 g finely ground soil (air-dry, < 0.25 mm) into a 60-ml platinum crucible.

Add approximately half of a weighed 2 g amount of anhydrous Na2C03 and stir thoroughly with a platinum wire or glass rod.

Cover the mixture with the rest of the Na2C03•

Place the covered crucible at a slight angle on a silica triangle over a Meker-type burner with a small diameter head.

Heat gently for about 5 min. until sample is molten, carefully lifting the lid to allow air in through a slit.

Increase the heat gradually until the full heat of the flame is being used. The bottom of the crucible should be a bright cherry red. Maintain this heat for 4-5 min. The crucible lid should be about one quarter removed to allow the admission of air, and for the last 2 min. removed completely.

In order to avoid reducing conditions during the fusion, it is most important to ensure that the flame from the burner does not completely envelope the crucible.

Remove the crucible and swirl until the melt has solidified around the sides of the crucible.

Cool, and transfer the crucible to a 250-ml beaker.

Holding the crucible lid with platinum-tipped tongs, wash any residue from its underside, with approximately 2 ml l + 3 H,SO, from a full 15-ml pipette, into the beaker.

Cover the beaker with a watch-glass and carefully add the remainder of the 15 ml l + 3 H2S04 into the crucible through the gap between the beaker spout and the watch-glass.

After the effervescence has completely subsided, tip the contents of the crucible into the beaker, and then remove the crucible after washing it thoroughly with hot distilled water.

Digest for l hour on a boiling water bath and filter through a no. 42 (Whatman) filter paper into a JOO-ml volumetric flask.

Wash beaker and filter paper into the volumetric flask with hot distilled water, cool, and make to volume.

A Na,CO, blank should be treated in the same way as the samples.

SFl .I : Manual Method

PREPARATION OF REAGENTS MURPHY AND RILEY REAGENTS A AND B. See Method SA.I.

PREPARATION OF STANDARDS WORKING STOCK (1 µg P/ml). See Method SA.I.

41

WORKING STANDARDS. Pipette 0, S, 10, lS, 20 and 2S-ml aliquots of standard phosphate solution (1 µg P/ml) into 100-ml volumetric flasks and dilute to approximately 30 ml with water. These solutions contain 0, S, 10, IS, 20 and 2S µg P respectively. Treat these solutions as for samples as regards pH adjustment and colour development.

PROCEDURE Pipette a suitable aliquot (usually 10 ml) of the sample solution into a 100-ml volumetric flask.

Adjust the pH, develop, and read the colour as described in Method SD.I.

CALCULATION OF RESULTS Prepare a standard curve from the readings for standards and read off the phosphorus concentrations for the unknowns in µg P per 100 ml.

P in final 100 ml (µg) volume (ml) ---~-~~x ~Total P (mg/lOOg)

10 weight soil (g) X aliquot (ml)

or, for a 0.2S g sample, making up to l 00 ml and taking a l 0-ml aliquot

P in final 100 ml (µg) X 4 ~ Total P (mg/100 g)


SFl.11 : AutoAnalyzer Set up AutoAnalyzer as described in Method SA.II, using appropriate standards for the range expected (normally 0-S µg P/ml).


For 0.2S g made to l 00 ml

µg P/ml X 40 ~ Total P (mg/100 g)


42

5F2 FUSION WITH NaOH

PREPARATION OF REAGENTS SULPHURIC ACID, CONC.

SODIUM HYDROXIDE PELLETS.

PROCEDURE Weigh 0.2S g finely ground soil (air-dry, < 0.2S mm) into a SO ml nickel crucible.

Add 2.S g NaOH pellets.

Place the crucible in a cool muffie furnace and heat to 800°C. When up to temperature, leave the samples for l S min. It is important to begin the fusion at a cooler temperature to prevent spattering.

Remove the crucibles and when cool place in 2SO-ml beakers. Fill with approximately 40 ml hot water. Leave the samples for about 30 min. or until the residue is in solution.

Wash the contents of the crucibles into the beakers with hot water and filter through no. 42 (Whatman) filter paper into 100-ml flasks.

Wash beaker and filter paper several times.

Add 4.S ml cone. H2SO, to each flask, cool and make to volume.

A NaOH blank should be treated in the same way as the samples.

NOTES I. A white precipitate of silica often forms when H,SO, is added. However, this does not affect total P results.

2. New nickel crucibles tend to leave a black nickel deposit in the filter paper after the first two or three fusions. No phosphorus is contained in the deposit.

5F2.I : Manual Method

PREPARATION OF REAGENTS MURPHY AND RILEY REAGENTS A AND B. See Method SA.I.

PREPARATION OF STANDARDS WORKING STOCK (1 µg P/ml). See Method SA.I.

WORKING STANDARDS. Pipette 0, S, 10, l S, 20 and 2S-ml aliquots of standard phosphate solution (l µg P/ml) into 100-ml volumetric flasks and dilute to approximately 30 ml with water. These solutions contain 0, S, 10, lS, 20 and 2S µg P respectively. Treat these solutions as for samples as regards pH adjustment and colour 'development.

PROCEDURE Pipette a suitable aliquot (usually 10 ml) of the sample solution into a 100-ml volumetric flask.

Adjust the pH, develop and read the colour as described in Method SD.I.

43

CALCULATION OF RESULTS Prepare a standard curve from the readings for standards and read off the phosphorus concentrations for the unknowns in µg P per 100 ml.

P in final I 00 ml (µg) volume (ml) 10 X weight soil (g) X aliquot (ml) ~ Total p (mg/100 g)

or, for a 0.25 g sample, making up to 100 ml and taking a 10-ml aliquot

Pin final 100 ml (µg) X 4 ~ Total P (mg/100 g)


5F2.II : AutoAnalyzer Set up AutoAnalyzer as described in Method 5A.II, using appropriate standards for the range expected (normally 0-5 µg P/ml).


For 0.25 g made to 100 ml

µg P/ml x 40 ~total P (mg/100 g)


44

5G PHOSPHATE RETENTION

PREPARATION OF REAGENTS PHOSPHATE-RETENTION SOLUTION (!mg P/ml). Dissolve 8.80 g potassium dihydrogen phosphate (KH2PO,), and 32.8 g anhydrous sodium acetate (CH,COONa) in water, add 23 ml glacial acetic acid, and dilute to 2 litres in a volumetric flask. The pH of this solution should be 4.6 ± 0.05.

NITRIC VANADOMOLYBDATE ACID REAGENT. Vanadate solution-dis-. solve 0.8 g ammonium vanadate (NH4V03) in 500 ml boiling water, cool the solution, add 6 ml cone. HN03 and dilute to 1000 ml with water. Molybdate solution-dissolve 16 g ammonium molybdate ((NH4) 6Mo70 24.2H20) in water at 50°C, cool and dilute to 1000 ml. Prepare dilute HN03 by diluting 100 ml cone. HN03 to 1000 ml with water. To this add first the vanadate solution and then the molybdate solution. Mix well.

PREPARATION OF STANDARDS WORKING STOCK SOLUTIONS. Pipette 0, 10, 20, 30, 40 and 50-ml aliquots of phosphate-retention solution (1 mg P/ml) into 50-ml flasks and make to volume with water. These solutions contain 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mg P/ml and correspond to I 00, 80, 60, 40, 20 and 0 percent retention respectively.

PROCEDURE Weigh 5 g soil (air dry, < 2 mm) into a stoppered, 50-ml polypropylene centrifuge tube, and add 25 ml P-retention solution.

Shake for 16 hr at about 20°C (about 50 r.p.m.).

Centrifuge at 2000 r.p.m. for 15 min.

Dilute supematants and working stock solutions 1 + 19 with the nitric vanadomolybdate acid reagent. Shake well.

Read the absorbance, after at least 30 min. at 466 nm.

CALCULATION OF RESULTS Prepare a standard curve of% phosphate retention against absorbance.

Read off unknowns directly as % phosphate retention (% P retn).

No moisture correction is made.

REFERENCES BALLARD. R. 1974: Use of soil testing for predicting phosphate fertiliser requirements

of radiata pine at time of planting. N.Z. Journal o.f Forestry Science 4: 27-34.

BRAY. R. H.; KURTZ, L. T. 1945: Determination of total. organic, and available forms of phosphorus in soils. Soil Science 59: 39-45.

COL WELL. J. D. 1965: Determination of phosphorus in sodium hydrogen carbonate extracts of soils. Che1nist1:i' and Indust1)' 21: 893-895.

CORNFORTH. I. S. 1980: Soils and Fertilisers: Soil Analysis: Interpretation. Ag Link FPP 556. NZ Ministry of Agriculture and Fisheries.

COWLING, J. C.; SPEIR, T. W.; PERCIVAL, H. J. In prep.: Potential problems with the determination of Olsen and microbial P due to the instability of0.5 M NaHC03.

45

MURPHY, J.; RILEY, J. P. 1962: A modified single solution method for the determination of phosphate in natural waters. Analytica Chimica Acta 27: 31-36.

OLSEN, S. R.; COLE, C. V.; WATANABE, Y S.; DEAN, L. A. 1954: Estimation of available phosphorus in soils by extraction with sodium bicarbonate. USDA Department Circular 939.

SAUNDERS, W. M. H. 1965: Phosphate retention by New Zealand soils and its relationship to free sesquioxides, organic matter and other soil properties. NZ Journal of Agricultural Research 8: 30-57.

SAUNDERS, W. M. H. 1968: Phosphorus. Chapter 7.7 pp. 95-102 in 'Soils of New Zealand, Part 2'. NZ Soil Bureau Bulletin 26(2). 221 p.

SMITH, B. F. L.; BAIN, D. C. 1982: A sodium hyroxide fusion method for the determination of total phosphate in soils. Communications in Soil Science and Plant Analysis 13: 185-190.

SYERS, J. K.; WILLIAMS, J. D. H.; WALKER, T. W. 1968: The determination of total phosphorus in soils and parent materials. NZ Journal of Agricultural Research 11: 757-762.

TRUOG, E. 1930: The determination of readily available phosphorus of soils. Journal of the American Society of Agronomy 22: 874-882.

WATANABE, F. S.; OLSEN, S. R. 1965: Test of an ascorbic acid method for determining phosphorus in water and NaHCO, extracts from soil. Soil Science Society of America Proceedings 29: 677-678.

6: CATION EXCHANGE PROPERTIES

47

Cation exchange is the interchange between a cation in solution and another cation on the surface of any surface-active material such as clay or organic matter. In soils the principal cations found in exchangeable forms are the exchangeable bases Caz+, Mgz+, Na+ and K+, although other cations such as aluminium, iron, H + and NH, + may be present.

The determination of exchangeable bases is useful for two main reasons. Firstly, exchangeable cations are considered to be available to plants by exchange with hydrogen ions from root hairs and soil micro-organisms. Therefore the relative amounts of such cations are important for plant nutrition. Secondly, the movement of cations within the profile is of importance in the dynamic approach to pedology and therefore ultimately to soil classification. The 'basic' cations may be leached down the soil profile by percolating rainwater, so in areas of moderate to heavy rainfall the overall process is one of depletion of the exchangeable bases of the soil.

The cation exchange capacity (CEC) of a soil is usually defined by the amount of a cation (such as NH, +, Baz+) that a soil can hold when a buffered or unbuffered salt solution is leached through the soil. The most widely used salt is ammonium acetate buffered at pH 7, but BaCl2 buffered at pH 8.2 is also used. 1 M potassium chloride and 0.01 M silver thiourea are examples of unbuffered solutions used. Results obtained by these methods may vary considerably, depending on the variable charge characteristics of the soil. The variation in values between methods is related to the pH and ionic strength of the reagents used.

Another measure of exchange capacity is 'Effective Cation Exchange Capacity' (ECEC). This is an estimate of the total amount of exchangeable cations the soil is actually holding at field pH, and is normally determined by adding the sum of the exchangeable bases to the amount of aluminium replaced by I M potassium chloride. (The four exchangeable bases plus aluminium represent virtually all the exchangeable cations normally present in soil).

The concept of soil cation exchange capacity has always been linked with that of the percentage saturation of this capacity with exchangeable bases, i.e., percentage base saturation (% BS). As there are a number of different definitions of cation exchange capacity, the % BS of a soil will depend on the method used.

Four methods for determining cation exchange properties are given in this chapter.

Method 6A is the I M ammonium acetate (pH 7) method, which is currently the most widely used procedure and is the one used for criteria in some classification systems.

Method 6B is a procedure for determining ECEC.

Method 6C is a procedure for estimating CEC at pH 8.2 from exchangeable bases and the ~xchangeable acidity extracted froni a soil at pH 8.2.

Method 6D is a rapid complete analysis scheme for cation exchange properties at field pH and low ionic strength, using silver thiourea solution.

48

6A CATION EXCHANGE PROPERTIES DETERMINED BY 1 M AMMONIUM ACETATE (pH 7)

Ammonium acetate is the salt most often used because it is the neutral salt of a weak acid and a weak base. These properties are desirable to ensure that exchange takes place at pH 7 and that hydrolysis during the exchange reaction is minimised. pH 7 was chosen for the measurement of cation exchange properties partly because this value represents 'true neutrality' and was considered to be near the optimum pH for agricultural soils. (See Schollenberger and Simon (1945) for more details).

Ammonium acetate is particularly suitable for a leaching procedure. It does not disperse clay soils, the NH/ ion, as the displacing cation, is rarely present in significant amounts in the exchange complex and is easily measured in the laboratory, and acetates of the basic cations are readily soluble.

6Al : EXCHANGE METHODS

PREPARATION OF REAGENTS ACID-WASHED SILICA SAND. Silica sand to be treated should be relatively pure (white) and should be sieved so that it is in the 0.25-0 .. 5 mm size range (to pass through a 30-60 mesh sieve). Place about 10 litres of this material into a suitable container (e.g., lixiviating beaker, chromatography tank), wash with tap water to disperse organic matter, silt, and clay, and after vigorous stirring, remove these impurities by decanting. This process should be repeated until the supernatant water is clear. Add 2.5 litres cone. HCl to the sand and leave overnight. Stirring vigorously, wash out the acid from the sand with copious quantities of tap water. Using a large Buchner funnel (with a Whatman no. 542 filter paper) connected to a vacuum pump, wash portions of the sand with distilled water until the filtrate is chloride-free. Dry in an oven.

MACERATED FILTER PAPER. Macerate by means ofa suitable machine (e.g., food blender), a quantity of fast-filtering, acid-washed filter paper (e.g., Whatman no. 31 or 41) in water. Keep moist in a closed jar.

AMMONIUM ACETATE (CH3COONH4), 1 M, pH 7.0. This is best prepared from glacial acetic acid and strong ammonia solution as it is cheaper, and the results for blank solutions are usually lower, than when it is prepared from the solid salt.

Prepare as follows: Add 1150 ml acetic acid (CH,COOH) (about 99%) to about 15 litres water, shake, and add 1500 ml ammonium hydroxide (NH40H) solution (sp. gr, 0.91). Make up to 20 litres and mix by shaking. Check pH and adjust to pH 7.0 ± 0.05 with 2 M NH,OH and 2 M CH3COOH. Allow to cool before using.

WASH ETHANOL (WASH ALCOHOL). Dilute ethanol (L.R. or commercial grade) to 90% w/w with water, using a hydrometer to check strength (water required= 9.7 ml water/litre alcohol/percentage change, e.g., 20 litres of 95% alcohol requires 970 ml water). Add 0.25 ml of 2 M NH,OH per litre. Determine the ammonium content by distillation of a 10-ml aliquot with NaOH (steam distillation as for nitrogen determination), and titrate the distillate with 0.020 M

HCI. Using bromocresol green-methyl red indicator, the titration should take between 0.18 and 0.33 ml HCI.

SODIUM CHLORIDE, approximately 1 M. Dissolve 1170 g NaCl in 20 litres water.

6Ala Macro leaching

PROCEDURE

49

Mix 5 g soil (air-dry, < 2 mm) with 10 g acid-washed silica sand and pack into a leaching tube (see Fig. 5) which has a macerated filter paper plug.

Percolate the soil with 230 ml 1 M ammonium acetate, pH 7.0, into a 250-ml volumetric flask and make to volume with water. Leaching time should be at least one hour, or low results may be obtained.

Carry out a blank determination on 10 g acid-washed silica sand.

' Rinse the top of t~:eaching tube and soil several times with small amounts of wash ethanol, allo~~g time to drain between each washing.

Percolate 200 ml ethanol through the tube and discard the percolate.

Place a 250-ml volume~fic flask under the leaching tube, and leach the soil column with 230 ml 1 M NaCl solution.

Make the solution to 250 mLwith water.

NOTE: I. Avoid formation of channels and air-locks in the column of soil and sand during leaching by applying suction to the outlet of the leaching tube when the ammonium acetate is first added. The suction must be gentle, and may be generated by attaching a short length of rubber tube to the outlet, squeezing the whole tube and, while blocking off the end, releasing the squeezed portion. The suction so obtained is applied until the ammonium acetate just reaches the filter paper plug, at which time the end of the rubber tube is released and the rubber tube quickly removed. The outlet of the leaching tube is then placed into the neck of the receiving volumetric flask during leaching. 2. In order to avoid contamination with sodium, a separate set of glassware should be used for the sodium chloride leaching. 3. A further leaching of the sample may be made to extract oxalateSciluble iron, aluminium, manganese and silicon (see Method SA).

6Al b Semi-micro leaching

PROCEDURE Mix 1 g of soil (air-dry, < 2 mm) with 2 g of acid-washed silica sand and pack into a semi-micro leaching tube (see Fig. 5) which has a macerated filter paper plug.

Carry out a blank determination on 2 g acid-washed silica sand.

Percolate the soil with 50 ml 1 M ammonium acetate, pH 7.0, into a 50-ml flask.

Make to volume with water.

Rinse the top of the leaching tube and soil several times with small amounts of wash ethanol, allowing time to drain between each washing.

Percolate 40 ml wash ethanol through the tube and discard percolate.

Place a 50-ml volumetric flask under the leaching tube, and leach with 45 ml 1 M NaCl.

Make to volume with water.

NOTE: The soil:solution ratio used here (1:50) is the same as Method 6Ala.

E 0

"'

E 0

CD

E ell

50

MACRO

24mm o.d. light wall

(Jobling 5500/24)

Bmm o.d. light wall

(Jobling 5500/08)

E 0

L!)

E 0

"'

E 0 ....

SEMI-MICRO

~ \ I v

15mm o.d. light wall

(Jobling 5500/15 )

7mm o.d. light wall

(Jobling 5500/07)

Figure 5 Leaching tubes (actual size)

51

6Alc Automatic Extractor This procedure is essentially a controlled leaching, using an extractor manufactured by Concept Engineering Corp. (USA). Solutions are pulled through the soil at a controlled rate using syringes-the 'reservoir', 'leaching tube' and 'receiving vessel'. are all whole, or part, syringes.

The procedure is particularly useful when it is essential to control the rate of leaching or when impermeable materials, such as swelling clays, have to be leached.

The following procedure is adapted from the methods outlined in. the manufacturers manual.

PROCEDURE Insert macerated filter paper wads into the leaching tubes and press flat (see Fig. 6).

Weigh 2.5-g samples ( < 2 mm) plus 5 g silica sand into 50-ml beakers and mix.

Use 5 g silica sand as a blank.

Transfer to leaching tubes, ensuring that the soil does not stick to the sides.

Place on apparatus and fit receiving syringes.

Add 1 M NH40Ac (ammonium acetate) to 20-ml mark in leaching tubes.

Let stand for 15-20 min.

Fit reservoirs to leaching tubes.

Extract rapidly until about 1 cm depth of NH40Ac remains above sample.

Add 45 ml 1 M NH,OAc to reservoirs.

Run extractor to complete extraction in 10-12 hrs.

Pull remaining NH,OAc through by hand pressure on syringe.

Transfer leachate to 100-ml volumetric flasks, rinsing with small portions of water.

Make to 100 ml with water for exchangeab;e base determinations.

Remove reservoirs and rinse with water.

Fit receiving syringes.

Rinse sides of leaching tubes with approximately 20 ml wash ethanol.

Run extractor until about 1 cm of wash ethanol remains above sample.

Fit reservoirs, add 50 ml wash ethanol and run extractor so that all the wash ethanol is extracted in about 90 min.

Discard ethanol leachate and refit receiving syringes.

Run extractor with a further 45 ml wash ethanol for about 90 min.

Discard leachates and wash receiving syringes.

Add 20 ml 1 M NaCl to leaching tube and rapidly extract until liquid is 1 cm deep.

Fit reservoirs and add 45 ml 1 M NaCL

Run extractor to complete extraction in 10-12 hrs.

Transfer NaCl leachates to 100-ml volumetric flasks and make to 100 ml with water for ammonium determination (CEC; Method 6A4).

52

Reservoir

... _ .. _ .. _

~---Extractant

Leaching tube

·---·-::.:· 1----Extractant l~~~;~~:t<'t----Sample ,>,,.;.,-.

. --~ r- --.

Filter pulp

Receiving syringe

Figure 6 Arrangement of syringes on automatic extractor

6A2 : INDIVIDUAL EXCHANGEABLE BASES

PREPARATION OF REAGENTS

53

STRONTIUM CHLORIDE-CAESIUM CHLORIDE SOLUTION. (7500 µg Sr/ml, 25 OOO µg Cs/ml). Weigh 46 g SrCl2.6H20, and 63.6 g CsCl, into a beaker, dissolve and make to 2 litres with water. This solution contains strontium to eliminate possible phosphate interference during the flame emission determination of calcium. Caesium is added to eliminate ionisation of exchangeable cations in the air/acetylene flame.

1 + 3 DILUENT SOLUTION. Carefully add 54 ml cone. HCl to about 200 ml water, add 213 ml SrC12/CsCl solution and make to 2 litres with water.

1 + 9 DILUENT SOLUTION. Carefully add 45 ml cone. HCl to 233 ml, I M

ammonium acetate, add 180 ml SrC12/CsCl solution and make to 2 litres with water.


PREPARATION OF STANDARDS CONCENTRATED STOCK SOLUTIONS

CALCIUM, 2.000 N solution: carefully dissolve 50.045 g calcium carbonate (CaC03), dried at 110°C, in enough I + l HCl to just dissolve it (about 200 ml), and make up to 500 ml with 0.2 M HCl.

MAGNESIUM, 0.2000 N solution: carefully dissolve 1.216 g magnesium ribbon in I + l HCI, as for calcium, and make to 500 ml with 0.2 M HCI.

POTASSIUM, 0.2000 N solution: dissolve 7.455 g potassium chloride (KC!), dried at l l0°C, in 0.2 M HCl, and make to 500 ml with 0.2 M HCI.

SODIUM, 0.2000 N solution: dissolve 5.845 g sodium chloride (NaCl), dried at l l0°C, in 0.2 M HCl, and make to 500 ml with 0.2 M HCI.

MULTIPLE WORKING STOCK SOLUTION. Pipette 25 ml of each concentrated stock solution into a 1000-ml volumetric flask, and make to mark with water. Store in an inert plastic bottle (preferably teflon).

WORKING STANDARDS. Pipette out aliquots of the multiple working stock solution to 500-ml volumetric flasks, according to the following table:

Working Volume of multiple Concentration of working standard working stock standards (10-4 N)

(ml per 500 ml) Ca Mg K Na

1 0.0 0.0 0.0 0.0 0.0 2 2.5 2.5 0.25 0.25 0.25 3 5.0 5.0 0.50 0.50 0.50 4 10.0 10.0 1.00 1.00 1.00 5 15.0 15.0 1.50 1.50 1.50 6 25.0 25.0 2.50 2.50 2.50 7 50.0 50.0 5.00 5.00 5.00

To each flask add 10 ml cone. HCl, 40 ml strontium-caesium solution (7500 µg Sr/ml, 25 OOO µg Cs/ml) and 125 ml I M ammonium acetate. Make up to 500 ml with water and mix well. Store in plastic bottles.

PROCEDURE Dilute sample and blank solutions, using the l + 3 diluent solution.

54

Determine exchangeable bases by flame spectrometry, using the working standards for comparison. Normally standard number 5 is used as the top standard, but if sample concentrations exceed this standard, standards 6 and 7 may be used to avoid further dilution of the sample. If further sample dilution is necessary, perform a 1 + 9 dilution using the appropriate diluting solution. If calcium values are less than 10 me./ 100 g, read using standard 3 as the top standard for calibration (0-10 me./ 100 g).

CALCULATION OF RESULTS Prepare a standard curve from the readings for the standards in each case (i.e., Ca*, Mg, K, and Na) and read off concentrations as N X 10-4•

For a 1:50 soil:solution ratio (Methods 6Ala & b) and

For a 1 + 3 dilution

N x 10-4 x 2 = me./100 g


N x 10-4 x 5 = me./100 g

For a 1:40 soil:solution ratio (Method 6Alc-Automatic extractor) and a 1 + 3 dilution

N x 10-4 X 1.6 = me./100 g

Apply blank correction, then use moisture factor (Method 1) to correct to ovendry basis.

*See Method 1 OD for correction needed when free CaC03 is present.

6A3 TOTAL EXCHANGEABLE BASES (TEB)

6A3a By titration (TEB,i10)

PREPARATION OF REAGENTS

55

METHYL RED INDICATOR, 0.1 %. Weigh 1.0 g finely ground methyl red powder into a 1500-ml beaker. Add 964 ml 95% ethanol and 36 ml 0.1 M NaOH. Stir until all the methyl red has dissolved.


SODIUM HYDROXIDE, 0.020 M.

PROCEDURE Pipette a 100-ml aliquot of the ammonium acetate leachate into a silica basin, evaporate to dryness, and heat for 30 min. at 500°C in a furnace.

Add a known amount of 0.050 M HCI (usually 10.0 ml), and digest on a waterbath for 30 min.

Back titrate with 0.020 M NaOH while still hot, using methyl red as indicator. If titration is less than 5 ml, add another 10.0 ml 0.050 M HCl, redigest, and repeat titration, summing the titrants.

This procedure is basically that described by Metson (1956) except that the reagents are weaker and therefore NaOH is preferable to NH,OH.

CALCULATION OF RESULTS [0.0500 M HCI added (ml) X 2.5] - 0.0200 M NaOH required (ml) ~

TEB (me./100 g)


6A3b By summation (TEBsum. or L Bases) This value represents the sum of the individual exchangeable bases (i.e., Ca, Mg, K, Na) when each is expressed as me./100 g. For the usual soils of humid regions, TEB,;m closely corresponds to TEB,0 m.· However, when soluble salts are present and are not removed before determination, TEB,0 m. will exceed TEB,;m by approximately the equivalent of the metal cations present as neutral salts. When calcium carbonate is present some is dissolved by the ammonium acetate leaching solution, but this is titratable by the TEB,;m procedure, so that both the apparent TEB,;m and the apparent TEB,0 m. are increased by an amount equivalent to the amount of calcium carbonate dissolved.

In very acid soils much aluminium, replaceable by ammonium acetate, may be present. As this aluminium is associated with the sulphate ion, an acid reaction is developed on ignition of the aluminium sulphate formed on evaporation, reducing the apparent TEB,;,, figure and, in extreme cases, negative values for TEB,;m may even be obtained (Blakemore 1964).

Apart from such exceptions, a comparison of TEB,;m with TEB,,m. provides a useful guide to the accuracy of the analysis as a whole.

CALCULATION OF RESULTS Ca + Mg+ K + Na ~ TEB,.m. (me./100 g)

56

6A4: CATION EXCHANGE CAPACITY (CEC)

6A4.I : Distillation

PROCEDURE Transfer a 20-ml aliquot of the 1 M NaCl leachate to a suitable ammonia distillation apparatus, using about 5 ml 1 + 2 NaOH to drive off the ammonia. See Method 4A.I for details.

Collect the ammonia in about 10 ml 1% boric acid and titrate with 0.020 M HCI using bromocresol green-methyl red indicator.

CALCULATION OF RESULTS For a 1:50 soil:solution ratio and 20-ml aliquot

0.020 M HCl used (ml) X 5 ~ CEC (me./100 g)


6A4.II : Autoanalyzer

The colorimetric determination of ammonium-nitrogen as described in Method 4A.II is used to determine the ammonia present in the 1 M NaCl leachates obtained by Method 6Al. This procedure is an alternative to the distillation method.

PREPARATION OF REAGENTS NaDTT, see Method 4A.II.

SODIUM NITROPRUSSIDE, see Method 4A.IL

SODIUM SALICYLATE, see Method 4A.II.

WASH SOLUTION, water plus a few drops of Brij 35 Wetting Agent.

CITRATE-TARTRATE, see Method 4A.II.

PREPARATION OF STANDARDS STOCK SOLUTION (700 µg NH4-N/ml). Dissolve 3.3011 g (NH4),SO, (M.A.R. or Aristar quality dried at l l0°C) in 1 M NaCl and make to 1 litre with 1 M

NaCl.

WORKING STANDARDS. Pipette 5, 10, 20, 30, and 40-ml aliquots of stock solution into 250-ml volumetric flasks, add 10 ml 90% ethanol, and make to mark with 1 M NaCl. These solutions then correspond to 14, 28, 56, 84 and 112 µg NH4-N/ml respectively. Ethanol is added to the standards because the samples contain approximately 10 ml of residue wash ethanol from the leaching procedure. This ethanol has a slight effect on the colorimetric determination.

PROCEDURE Pour NaCl leachate solutions into AutoAnalyzer sample cups.

Set up manifold as in Method 4A.II (see Figure 2), but use water as wash solution. Set the recorder baseline using the colorimeter baseline control.

Sample 112 µg NH4-N/ml standard, and when it reaches the colorimeter flowcell, set the recorder to read 100% absorption, using the standard calibration control on colorimeter. Reset baseline if necessary.

57

Sample standards and unknowns at 60/hr with a 5: 1 sample:wash ratio.

Samples containing more than 112 µg NH4-N/ml can be diluted using 1 M NaCl as diluent.

CALCULATION OF RESULTS For 1:50 soil:solution ratio the standards are equivalent to the following CEC (me./ 100 g) values:

Standard CEC (µg NH,-N/ml) (me./100 g)

112 40.0 84 30.0 56 20.0 28 10.0 14 5.0

Construct a standard curve of CEC (me./ 100 g) against abscirbance for the standards (this should be near linear).

Read off unknowns directly as me./ 100 g.


NOTE: For a 1:40 soil:solution ratio (Method 6Alc) multiply the final answer by 0.8.

6A5 : CALCULATION OF % BS (pH 7) This value is derived from the CEC and TEBrnm. (or sometimes TEB,.'") results, when both are expressed as me./100 g, thus,

TEB,0 m (me./100 g) X 100 ~ % BS (pH ?) CEC (me./ 100 g)

58

6B: EFFECTIVE CATION EXCHANGE CAPACITY (ECEC)

The concept of this method is discussed in the introduction to this section. The method is a summation method for measuring CEC and is used extensively in Soil Taxonomy (Soil Survey Staff 1975) (e.g., in the definition of oxic horizons).

6Bl 1 M KCl-EXTRACTABLE ALUMINIUM

PREPARATION OF REAGENTS POTASSIUM CHLORIDE, 1 M. Dissolve 373 g potassium chloride in water and make to 5 Ii tres.

PREPARATION OF STANDARDS STOCK SOLUTION (200 µg Al/ml). Dissolve 0.200 g pure aluminium wire in about 5 ml cone. HCI with the addition ofa trace (one single crystal) ofa soluble mercury salt to catalyse the reaction. Make to 1 litre with water. Alternatively, dilute 1 OOO µg/ml commercial stock solution.

WORKING STANDARDS. Pipette 0, 5, 10, 25 and 50 ml of stock solution (200 µg Al/ml) into 20o:m1 volumetric flasks, add 50 ml of l M KC!, l ml cone. H,SO,, and then make to 200 ml with water. These solutions then contain 0, 5, 10, 25 and 50 µg Al/ml respectively.

PROCEDURE Weigh 5.0 g of soil (air-dried, < 2 mm) into a 125-ml conical flask.

Add 25 ml of I M KC! and swirl gently to saturate soil.

Allow to stand for about 16 h (overnight).

Filter through a no. 42 (Whatman) filter paper into a 200-ml volumetric flask.

Wash the remaining soil in the conical flask and filter paper with a further 25 ml of 1 M KC!.

Wash soil and filter paper twice with water.

When filtered, add I ml cone. H2S04 and make up to volume with water.

Include a reagent blank in the procedure.

Determine aluminium in extracts directly by high-temperature flame spectrometry.

Note. H2SO, is added to extracts to prevent biological growths.

CALCULATION OF RESULTS Prepare a standard curve of µg Al/ml against % transmission or absorbance.


µg Al/ml X 0.444 = Al (me.j I 00 g)


6B2: CALCULATION OF ECEC

KCl-extractable Al (me.flOO g) + E bases (me./100 g) ~ ECEC (me./100 g)

NOTE: E bases as calculated in Method 6A3.

6B3 : CALCULATION OF % BS (ECEC) (if required)

E bases (me./ 100 g) X 100 ~ % BS (ECEC) ECEC

59

60

6C: CATION EXCHANGE CAPACITY AT pH 8.2 Cation exchange capacity can be measured at pH 8.2,· which represents the pH of a base-saturated soil in equilibrium with excess CaCO,, and with a C02 partial pressure equal to that in the atmosphere. It also represents a pH at which Ba absorption attains a maximum and hence CEC reaches a point of relative constancy (Melich 1942).

The value is riot measured directly, but is estimated by adding exchangeable bases to exchange acidity. This latter property refers to the acidity measured by releasing aluminium and displaced hydrogen at pH 8.2, neutralising the resulting acidity with triethanolamine (TEA), and titrating the excess TEA with standard acid.

The base saturation of the CEC at pH 8.2 is used as a criterion in Soil Taxonomy (e.g., for separating Alfisols and Ultisols).

6Cl : EXCHANGE ACIDITY The method described is adapted from USDA Soil Survey Investigations Report No. 1 (1972) and offers advantages in reproducibility and convenience. The main features of the modified method are the use of overnight shaking rather than leaching and the titration of an aliquot of the equilibrated buffer solution rather than the whole solution. This avoids the need to recover the remaining TEA completely by leaching with replacement solution, or with further portions of buffer solution.

PREPARATION OF REAGENTS TRIETHANOLAMINE BUFFER SOLUTION. To 61.07 g BaCl,.2H20 add 28 ml TEA and make to 1 litre with water. Adjust the pH of the solution to 8.2 with 1 + 1 HCl (usually 10 ml per litre). Protect from CO, by attaching a drying tube containing a C02 a)Jsorbant (e.g., soda lime) to the air intake.


BROMOCRESOL GREEN, 0.1% aqueous solution.

MIXED INDICATOR. Dissolve 1.250 g methyl red and 0.825 g methylene blue in 1 litre 90% ethanol.

PROCEDURE Weigh 2.5 g soil (air-dry, < 2 mm) (1.0 g for soils high in organic matter or amorphous oxides) into 50-ml, screw-cap, polypropylene centrifuge tubes.

Add 25 ml of buffer solution and shake overnight (16 h) on an end-over-end shaker (about 50 r.p.m.).

Centrifuge at 2000 r.p.m. for 15 min.

Shake at least two blanks (i.e., 25 ml buffer solution).

Transfer 10.0-ml aliquot to 125-ml conical flask and add about 20 ml water.

Add 1 drop bromocresol green and 5 drops mixed indicator.

Titrate with 0.10 M HCl until solution reaches first full purple colour.

Titrations on blanks should be carried out first and unknowns titrated to the same point of colour change.

CALCULATION OF RESULTS Using 1:10 soil:solution ratio

[ml 0.10 M HCI (blank)- ml 0.10 M HCI (sample)] x 10 ~ Exchange acidity (me./100 g)

Using 1 :25 soil:solution ratio

[ml 0.10 M HCl (blank) - ml 0.10 M HCl (sample)] X 25 ~ Exchange acidity (me./100 g)


6C2: CALCULATION OF CEC AT pH 8.2

61

Exchange acidity (me./100 g) + L bases (me./100 g) ~ CEC (pH 8.2) (me./100 g)

NOTE: L bases as calculated in Method 6A3.

6C3: CALCULATION OF 0/o BS AT pH 8.2

L bases (me./100 g) X 100 ~ % BS (pH 8_2) CEC (pH 8.2) (me./100 g)

62

measure exch. Ca, dilute aliquot

Mg, Na, K by 1+3

flame spectrometry

SOIL: EXTRACTANT 0.B g : 40 ml

16 h shake

centrifuge

decant 25 ml

SUPERNATANT~~d_i_lu_~~al_iq_u_ot~__... about BOX

add 0.1 m I saturated

KC I to remaining supernatant

i measure exch. Al and Mn

by AA.S.

measure Ag by AA.S.

to get CEC

Figure 7 Flow diagram of Ag TU method for determination of cation exchange properties

6D CATION EXCHANGE PROPERTIES DETERMINED BY 0.01 M SILVER THIOUREA

63

CEC methods that use reagents of high pH ( > 7.0) and high electrolyte concentrations overestimate values of CEC for variable-charge soils (Gillman 1979). To overcome this problem, methods that use low electrolyte concentrations and unbuffered salts have been used (Greenland 1974; Gillman 1979). However, the problem with such methods is that they require time-consuming equilibrations. The single extraction silver thiourea (AgTU) method for measuring exchangeable cations and effective cation exchange capacity (ECEC) uses an unbuffered 0.01 M

solution of an AgTU complex (Pleysier and Juo 1980). This complex ion is a very efficient exchanger of cations on clay surfaces, and exchange takes place at low ionic strength (I= 0.01), similar to that of many soil solutions (Gillman 1979; Edmeades 1983), and time-consuming equilibrations are not necessary. The procedure does not involve the displacement of an adsorbed index cation and so avoids the problems associated with hydrolysis or salt adsorption which can occur with other methods (e.g., leaching with ammonium acetate). The method therefore combines all the theoretically desirable features of a CEC method with the convenience of a simple, single extraction. Pleysier and Juo (1980) recommend the method for routine use on tropical soils, and it also has been evaluated on a wide range of New Zealand soils (Searle 1986).

A flow diagram of the whole procedure is shown in Fig. 7.

6Dl : EXTRACTION

PREPARATION OF REAGENTS SILVER THIOUREA (AgTU) 0.01 M. Dissolve 150 g thiourea in about 3 litres of water in a 10-litre container. Slowly add 5 litres of water containing 16.99 g silver nitrate, stirring with a magnetic stirrer. Make to 10 litres with water.

PROCEDURE Weigh 0.80 g soil (air-dry, < 2 mm) into a 50-ml stoppered polypropylene centrifuge tube.

Add 40.0 ml of 0.01 M AgTU and shake on an end-over-end shaker for 16 h (overnight).

Centrifuge at 2000 r.p.m. for 10 min and decant 25 ml of clear supernatant into a graduated test tube.

Carry a reagent blank throughout procedure.

6D2 : INDIVIDUAL EXCHANGEABLE BASES

PREPARATION OF REAGENTS 1 + 3 DILUENT SOLUTION. Take 213 ml of SrClJCsCl solution (7500 µg Sr/ml, 20 OOO µg Cs/ml, see Method 6A2) and make to 2 litres with water . ... 1 + 9 DILUENT SOLUTION. Take 177 ml of SrCl2/CsCl solution (as above) and 333 ml of 0.01 M Ag TU solution and make to 2 litres with water.

CAESIUM CHLORIDE SOLUTION (1000 µg Cs/ml). Dissolve 6.3 g CsCI in water and make to 5 litres.

SATURATED POTASSIUM CHLORIDE (approx. 180 g KCl/500 ml).

64

PREPARATION OF STANDARDS MULTIPLE WORKING STOCK (see Method 6A2).

WORKING STANDARDS. Pipette 0, 1, 2, and 6 ml of multiple working stock solution into 200-ml flasks, add 50 ml 0.01 M AgTU solution and 16 ml SrCl2/CsCl solution and make to volume with distilled water. These solutions then correspond to 0, 0.25, 0.50 and 1.50 N X 10-• Mg, K and Na and 0.0, 2.5, 5.0 and 15.0 N X 10-• Ca.

PROCEDURE Dilute solution from Method 6D 1 with the 1 + 3 diluent.

Determine exchangeable bases by flame spectrometry, using the working stand.ards.

CALCULATION OF RESULTS Prepare a standard curve from the readings for the standards in each case (i.e., Ca, Mg, Na and K) and read off concentrations as N X 10-•.

When a 1:50 soil:extractant ratio is used

For a l + 3 dilution

N X 10-• x 2 ~ me./100 g


N X 10-• X 5 ~ me./100 g


6D3 CATION EXCHANGE CAPACITY

PREPARATION OF REAGENTS THIOUREA SOLUTION. Dissolve 30 g thiourea in water and make to 2 litres.

PREPARATION OF STANDARDS Into five 100-ml volumetric flasks place

I. l 00 ml thiourea

2. 25 ml AgTU and 75 ml thiourea



5. 100 ml AgTU.

These correspond to 0, 0.25 X 10-2 N, 0.50 x 10-2 N, 0. 75 X 10-2 N and 1.0 X 10-2 N AgTU.

PROCEDURE Dilute sample extracts (6Dl) and standards about 80 times with the 1000 µg Cs/ml solution (Method 6D2). (The exact dilution ratio is not important as long as it is the same with samples and standards). The large dilution is necessary for linear calibration when measuring Ag by atomic absorption spectrometry (AAS).

Determine Ag using an air/C2H2 flame and the 328.1 nm absorption line. The 338.3 nm line may also be used and gives about double the linear range.

65

Aspirate water for at least 5 minutes when finished, and clean burner and mixing chamber to prevent the accumulation of Ag acetylides which are potentially hazardous.

CALCULATION OF RESULTS Prepare a standard curve from the readings for the standards and read off concentrations as N X 10-2.


[1 - (N x 10-2)] x 50 ~ CEC (me,/100 g)


A reagent blank is not required because the standards are prepared by appropriate dilutions of the extracting solutions.

6D4 EXCHANGEABLE Al AND Mn

PREPARATION OF STANDARDS EXCHANGEABLE Al. Pipette 0, 2, 5 and 10 ml of 200 µg Al/ml stock solution (see Method 6Bl) into 200-ml volumetric flasks. Add 1 ml of saturated KC! and make to volume with 0.01 M AgTU. These solutions then correspond to 0, 2, 5 and 10 µg Al/ml and are equivalent to 0, I.I I, 2.78, and 5.56 me. Al/100 g when applied to the undiluted AgTU extract.

EXCHANGEABLE Mn. Pipette 0, 1, 2, and 5 ml of200 µg Mn/ml stock solution (prepared by 5 X dilution of 1000 µg Mn/ml stock solution) into 200-ml volumetric flasks. Add 1 ml of saturated KC! solution and make to volume with 0.01 M AgTU. These solutions contain 0, 1, 2 and 5 µg Mn/ml and are equivalent to 0, 0.182, 0.364 and 0.910 me. Mn/100 g when applied to the undiluted AgTU extract.

NOTE: Al and Mn standards develop a precipitate with time. If this occurs, they should be made freshly. The formation of the precipitate is reduced if the solutions are stored in the dark.

PROCEDURE After the dilutions have been made for exchangeable bases and CEC, add 0.1 ml saturated KC! to the remaining sample extract (approximately 20 ml) to suppress ionisation of Al and Mn in the flame.

Measure Al, using AAS and the N,0/C2H2 flame, and Mn by AAS with an air/C2H2 flame.

CALCULATION OF RESULTS Prepare a standard curve from the readings for the standards for each element and read off concentrations as µg/ml.

For a 1:50 soil:extractant ratio

for Al

µg/ml X 0.556 ~ me./100 g

for Mn

µg/ml X 0.182 ~ me./100 g


66

6D5 CALCULATION OF EFFECTIVE CEC This value is determined by summing all the measured exchangeable cations (:!: Ca + Mg + Na + K + Al + Mn) and should be in close agreement to the value obtained for CEC by measuring Ag (Method 6D3). If ECEC exceeds CEC, this may indicate the presence of soluble salts. If CEC exceeds ECEC, this may indicate the presence of other exchangeable cations (e.g., NH.+, H+) not measured by the normal procedures.

CALCULATION OF RESULTS :!: cations (Ca+ Mg + Na + K + Al + Mn) = ECEC (me./100 g).

6D6 CALCULATION OF 0/o BS(AgTU)

:!: bases (me./100 g) X 100 = % BS (AgTU) CEC (me./ 100 g)

NOTE: :!: bases as calculated in Method 6D2 and CEC as calculated in Method 603.

REFERENCES BLAKEMORE, L. C. 1964: Effect of adsorbed sulphate on the determination of total

exchangeable bases in soils. NZ Journal of Agricultural Research 7: 435-438.

EDMEADES, D. C. 1983: Some comments on the use of effective CEC as a measure of nett negative charge in soils. Soil News 31: 70-73.

GILLMAN, G. P. 1979: A proposed method for the measurement of exchange properties of highly weathered soils. Australian Journal of Soil Research 17: 129-139.

GREENLAND, D. J. 1974: Determination of pH-dependent charges of clays using CsCl and X-ray fluorescence spectrography. Transactions of International Congress of Soil Science 19th Congress. Moscow 2: 274-281.

MELICH, A. 1942: Rapid estimation of base exchange properties of soil. Soil Science 53: 1-14.

METSON, A. J. 1956: Methods of chemical analysis for soil survey samples. NZ Soil Bureau Bulletin 12: 208 p.

PLEYSIER, J. L.; JUO, A. S. R. 1980: A single-extraction method using silver-thiourea for measuring exchangeable cations and effective CEC in soils with variable charges. Soil Science 129: 205-211.

SCHOLLENBERGER, C. J.; SIMON, R. H. 1945: Determination of exchange capacity and exchangeable bases in soil-ammonium acetate method. Soil Science 59: 13-24.

SEARLE, P. L. 1986: The measurement of soil cation exchange properties using the single extraction, silver thiourea method: An evaluation using a range of New Zealand soils. Australian Journal of Soil Research 24: 193-200.

SOIL SURVEY STAFF 1975: 'Soil Taxonomy. A basic system of soil classification for making and interpreting soil surveys.' USDA. Agricultural Handbook 436. 754 p.

67

7 : RESERVE NUTRIENTS

7A RESERVE POTASSIUM (K.) This value, which was proposed by Metson et al. (1956), represents the longterm potassium-supplying power of the soil. The method disregards the more readily extractable fraction of the non-exchangeable potassium and represents the nearly constant rate of release after several extractions. This rate of release is a function of the mineral reserves present in the soil (Melson 1968) and is not affected by organic matter return, fertiliser applications, urine patches, etc.

In order to determine this value, the more readily soluble potassium is removed by a single extraction with 1 M HN03 at a wide acid:soil ratio. This fraction is discarded. Two or three successive extractions with 1 M HN03 at a narrower acid:soil ratio are then made and the K concentration in these extracts is determined. The results of these two or three extractions, which should be fairly similar, are averaged to give the K value.

PREPARATION OF REAGENTS NITRIC ACID, approximately 1 M. Dilute 318 ml cone. HN03, sp. gr. 1.42, to 5 litres with water.

CAESIUM CHLORIDE (20 OOO µg Cs/ml). Weigh 25.4 g CsCl into a beaker, dissolve and make to 1 litre with water. This solution is added to samples and standards to suppress potassium ionisation in the air/acetylene flame.

PREPARATION OF STANDARDS STOCK SOLUTION, 0.200 N K. See Method 6A2.

WORKING STOCK SOLUTION, 0.002 N K. Pipette 10 ml of stock solution (0.200 N K) into a I-litre volumetric flask and make to volume with water.

WORKING STANDARDS. Pipette 0, 5, 10, 15, 20, 30 and 50 ml aliquots of the working stock solution (0.002 N K) into 200-ml volumetric flasks, add 20 ml caesium chloride solution (2000 µg/ml Cs) and 100 ml of 1 M HN03 and make to volume. These solutions then correspond to 0, 0.5, 1.0, 1.5, 2.0, 3.0 and 5.0 N X lQ-4 with respect to potassium.

PROCEDURE Weigh out 2.00 g soil (air-dry, < 2 mm) into a 400-ml extraction flask, or a widemouth, conical flask.

Include a blank determination throughout the procedure.

Add 200ml l M HN03 and boil for 20 min with a smaller conical flask inverted in the neck to act as a reflux condenser.

Allow solution to stand until supernatant is clear (about 30 min).

Remove the bulk of the supernatant with a suitable suction device, taking care not to remove any soil. If solution does not clear on standing, wash the contents of the flask into a 250-ml centrifuge bottle and centrifuge for 15 min at about 2000 r.p.m. Pour off and discard the liquid.

68

With approximately 25 ml I M HN03, transfer soil to a 50-ml centrifuge tube, centrifuge, and discard liquid.

Transfer soil to a 100-ml conical flask with Bl9/26 Socket, using exactly 25 ml 1 M HN03 in about five portions (dispensed from a 25-ml burette).

Fit a Bl9/26 cone joint (about 110 mm in length) in the neck of the flask to act as an air condenser, and gently boil on a hot plate for 10 min (actual boiling time).

Transfer liquid to a 50-ml centrifuge tube (coarse sand etc. may be left in flask) with a very small amount of water, centrifuge, and pour liquid into a 50-ml volumetric flask. Add 5 ml of CsCl solution and make to volume with water.

Transfer soil to original 100-ml conical flask with a further 25 ml of 1 M HN03 and repeat this boiling and centrifuging process.

Repeat with a further 25 ml 1 M HN03 if a third extraction is to be made (normally two extractions give a sufficiently accurate result).

Determine potassium concentrations in the separate extractions by flame spectrometry.

CALCULATION OF RESULTS Prepare a standard curve of N X 10-4 K against % transmission or absorbance.

For 2 g made to 50 ml

mean N X 10-4 K 4

~ K (me./100 g)

69

7B RESERVE MAGNESIUM (Mgr) This value represents the acid-soluble 'reserve' magnesium of the soil (Metson and Brooks 1975). It is calculated by determining the magnesium-soluble in boiling I M HCI (Mg,J and subtracting the exchangeable magnesium- (Mg.,) obtained by leaching with I M ammonium acetate (see Method 6A2).

Thus

PREPARATION OF REAGENTS HYDROCHLORIC ACID, I M. Carefully add 430 ml of cone. HCl to 2 litres of water and make to 5 litres. Standardise with standard alkali of comparable strength.

1 + 9 DILUENT SOLUTION. Add 175 ml SrCl,/CsCl solution (see Method 6A2) and 16 ml cone. HCl, A.R., to 556 ml 1 M ammonium acetate and make to 2 litres with water.

PREPARATION OF STANDARDS WORKING STANDARDS. As for Method 6A2.

PROCEDURE Weigh 2.5 g of soil (air dry, < 2 mm) into a 400-ml extraction flask.

Include a reagent blank throughout the procedure.

Add JOO ml 1 M HCl and place an inverted 50-ml conical flask in the neck of the extraction flask to act as a condenser.

Place the extraction flasks on a preheated hotplate (the liquid should boil after approximately 10 min and then the hotplate heat should be reduced).

Boil for 15 min (use timer) with occasional swirling.

Cool for 10 min after removing from hotplate and rinse the condenser flask into the extraction flask.

Filter extract into a 250-ml volumetric flask through a no. 42 (Whatman) filter paper and wash four times with hot distilled water.

Allow the filtrate to cool before making to volume with water.

Dilute the sample solutions 1 + 9 with the diluent solution and determine magnesium by atomic absorption spectrometry at 285.2 nm using the exchangeable base standards (Method 6A2).

CALCULATION OF RESULTS Prepare a standard curve of N X lQ-4 against absorbance.

For 2.5 g soil made to 250 ml and for a 1 + 9 dilution

Mg (N X lQ-4) X 10 ~ Mg" (me./ 100 g)

Apply a blank correction, then use moisture factor (Method 1) to correct Mg,. to oven-dry basis.

Mg,. (me./100 g) - Mg,,(me./100 g) ~Mg, (me./100 g)

70

REFERENCES METSON, A. J.; ARBUCKLE, R.H.; SAUNDERS, M. L. 1956: The potassium-supplying

power of New Zealand soils as determined by a modified n-ormal-nitric-acid method. Transactions of the 6th International Congress of Soil Science B: 619-627.

METSON, A. J. 1968: The long-term potassium-supplying power of New Zealand soils. Transactions of the 9th International Congress of Soil Science 2: 621-629.

METSON, A. J.; BROOKS, Jean,.M. 1975: Magnesium in New Zealand soils. II. Distribution of exchangeable and reserve magnesium in the main soil groups. NZ Journal of Agriculture Research 18: 317-335.

8: EXTRACTABLE IRON, ALUMINIUM AND SILICON

•SA ACID OXALATE-EXTRACTABLE IRON, ALUMINIUM AND SILICON

71

The acid oxalate extraction method (Tamm 1922) depends mainly on the complexing affinity of acid oxalate to extract the Fe, Al, and Si from short-range order materials. Its attack on crystalline oxides and crystalline clay minerals is very limited (Tamm 1932), and because the forms which it dissolves play a major part in cation and anion retention and other surface phenomena, extraction with this reagent is useful in soil studies (Saunders 1965). The data can also be used to estimate the allophane and ferrihydrite contents of soils (Parfitt and Childs, in prep.).

Two extraction procedures are used:

1. A shaking method.

2. A leaching method (Daly and Binnie 1974). This is carried out after the leachings for determining cation exchange properties and thus avoids further weighings, shakings and filterings.

At NZ Soil Bureau the extraction solution proposed by Tamm (1922) has been used. This was a 0.175 M solution of ammonium oxalate buffered to pH 3.25 with oxalic acid, giving a total oxalate strength of0.275 M. To be consistent with international methods, it has been decided to change to the weaker extracting solution described by McKeague and Day (1966). This is a 0.2 M solution of ammonium oxalate buffered to pH 3 with 0.2 M oxalic acid giving a total oxalate strength of 0.2 M. It has been found that the stronger reagent (0.275 M) used at a 1:40 soil reagent ratio for extraction by leaching gives very similar values to a 1:100 ratio extraction by shaking, for a wide range of Al, Fe and Si values. However, when using the weaker reagent (0.2 M) for samples with relatively large amounts of short-range order materials ( > about 2% Al), it has been found that neither a 1:40 leaching nor a 1:40 shaking extract as much as the stronger reagent or a 1:100 shaking.

Therefore it is recommended that if high values ( > about 2% Al) are obtained or expected, the extraction be carried out by shaking at a I: 100 ratio. For lower values, the I :40 ratio leaching is preferred for analytical convenience.

PREPARATION OF REAGENTS ACID OXALATE REAGENT. Dissolve 162 g ammonium oxalate and 108 g oxalic acid in water and make to I 0 litres. Check that the pH is 3 ± 0.05.

CAESIUM CHLORIDE, 20 OOO µg Cs/ml. Dissolve 12.65 g CsCI in water and make to 500 ml.

I + 9 DILUENT SOLUTION. Make 1.41 g CsCl and 22 ml cone. HCl to I litre with water.

72

PREPARATION OF STANDARDS STOCK SOLUTION (1000 µg Fe, Al, Si/ml). Dissolve I.OOO g pure aluminium wire in 20 ml cone. HCI with the addition of a trace (one small crystal) of a soluble mercury salt to catalyse the reaction.

Dissolve I .OOO g pure iron wire in 40 ml I: l HCI. The dissolution is hastened if the wire is cut up into small pieces and the solution heated on a water bath.

NOTE: It may be necessary to clean the iron before weighing by immersing 1.5-2.0 g in cone. HCI until clean, washing with water and drying quickly.

Weigh 2.139 g pure silica sand (finely ground in an agate mortar and pestle and dried at 450'C) into a platinum crucible and add 12 g Na2C03• Heat over a burner till molten and transfer to a muf!le furnace at 1 OOO'C for 30 min. Cool, and dissolve the melt in about 200 ml water. If the melt solution is cloudy, discard and repeat the process with finer ground sand.

Add Al and Fe solutions to a 1000-ml volumetric flask, add approximately 500 ml water, then add Si solution slowly while stirring, and make to volume with water. Add about 1 ml of toluene as a preservative.

N.B. Commercially available 1000 µg/ml standards may also be used.

WORKING STANDARDS (Fe, Al and Si). Pipette 0, 1, 2, 5, 10, and 20 ml stock solution (1000 µg Fe, Al and Si/ml) into 200-ml flasks, add 10 ml CsCI solution (20 OOO µg/ml), 20 ml acid oxalate reagent, 4 ml cone. HCI and make to volume with distilled water. These contain 0, 5, 10, 25, 50 and 100 µg Fe, Al and Si/ml.

8A1 EXTRACTION BY SHAKING

PROCEDURE Weigh l.O g soil (air-dry, < 2 mm) into a 250-ml centrifuge bottle.

Add 100 ml acid oxalate reagent.

Shake for 4 h on an end-over-end shaker (about 50 r.p.m.). The extraction should be carried out in the dark.

Filter through a no. 42 (Whatman) filter paper and collect about 50 ml.


Dilute samples 1 + 9 using the 1 + 9 diluent.

Using the 0-50 µg/ml standards, this dilution ratio gives a range of 0-5% (Fe, Al, Si) in the soil.

Measure Al and Fe by atomic absorption spectrometry (AAS) or flame emission spectrometry (FES), using a lean nitrous oxide-acetylene flame and Si by AAS using a moderately rich nitrous oxide-acetylene flame (Searle and Daly 1977).

NOTE: A fuel-rich nitrous oxide-acetylene flame has a strong emission band near the Fe and Al wavelengths (370.0 nm to 390.0 nm), so it is necessary to use a lean flame to obtain a suitable signal:noise ratio for FES. However, a very lean flame is relatively cool and will cause a decrease in sensitivity. The ideal flame has a 'red feather' zone l-2 cm high.

73

CALCULATION OF RESULTS Prepare standard curves for µg/ml (Fe, Al or Si) against % transmission or absorbance.


µg/ml in final solution X dilution factor 100

= % Fe, Al or Si

i.e., for 10 X dilution (1 + 9)

µg/ml 10

= % Fe, Al or Si


8A2 EXTRACTION BY LEACHING

PROCEDURE After the various leachings for cation exchange properties (Method 6A 1 a), leach 190 ml acid oxalate reagent through the soil retained in the leaching tube into a 200-ml volumetric flask. The blank should be leached in the same way.

Make to volume with acid oxalate reagent.

Dilute samples 1 + 9 using the 1 + 9 diluent.

Using the 0-100 µg/ml standards, this dilution ratio gives a range of 0-4% (Fe, Al and Si) in the soil. Samples containing higher amounts than this should be re-extracted using the shaking method (see above).

Measure Al, Fe and Si as described in Method SA!.


Prepare standard curves for µg/ml (Fe, Al or Si) against % transmission or absorbance.

For a 1 :40 extraction ratio.

µg/ml in final solution X dilution factor 250

= % Fe, Al or Si

i.e., for 10 X dilution (I + 9)

µg/ml = % Fe, Al or Si

25 Apply a blank correction, then use moisture factor (Method 1) to correct to ovendry basis.

74

SB PYROPHOSPHATE-EXTRACTABLE IRON AND ALUMINIUM

Until recently it was considered that pyrophosphate extracted Fe and Al from organic complexes but only relatively small amounts from amorphous (shortrange order) minerals (McKeague 1967). However, recent literature indicates that while the reagent extracts Al from organic complexes, the Fe is extracted from ferrihydrite (Parfitt and Childs in prep.). Values from this method are used in a number of soil classification systems to identify podzolised soils.

In some published methods, a choice is offered for ways of cleaning the extracts before measurement. Some use high-speed (about 20 OOO r.p.m.) centrifugation while others use 'superfloc' followed by low-speed centrifugation (up to 2000 r.p.m.): It has been found that the latter method, when used on soils which have large amounts of amorphous material or finely divided secondary iron oxides, does not provide suitably clear extracts. This causes erroneously high values to be obtained by flame spectrometry. This problem has also been noted in Canada (Ballantyne et al. 1980). The procedure used at NZ Soil Bureau combines highspeed centrifugation (20 OOO r.p.m.) and the addition of'superfloc'. The presence of 'superfloc' helps avoid the resuspension of the solid material after centrifuging.

Extractable C may also be determined on these extracts (see Method 3C) which is required in Soil Taxonomy for spodic horizon definition where FeP is < 0.1%.

PREPARATION OF REAGENTS SODIUM PYROPHOSPHATE, 0.1 M. Dissolve 223 g Na4P20,.IOH20 in water and make to 5 litres.

'SUPERFLOC' SOLUTION, 0.2%.

PREPARATION OF STANDARDS STOCK SOLUTION (1000 µg Fe, Al and Si/ml, as in Method 8A).

WORKING STANDARDS. Pipette 0, 2, 5, 10, 15 and 20 ml stock solution into 200-ml volumetric flasks, add 40 ml 0.1 M sodium pyrophosphate solution and make to volume with water. These solutions contain 0, 10, 25, 50, 75 and 100 µg/ml Fe and Al.

PROCEDURE Shake 1 g soil (air-dry, < 2 mm) with 100 ml 0.1 M sodium pyrophosphate reagent in a 250-ml centrifuge bottle on an end-over-end shaker (about 50 r.p.m.) overnight (16 h).

Add 5 drops 0.2% superfloc and shake vigorously. Centrifuge at 20 OOO r.p.m. for 30 min.

Dilute 1 + 4 with water and read concentration of Fe and Al by high-temperature FES or AAS.


CALCULATION OF RESULTS Prepare standard curves for µg Fe or Al/ml against % transmission or absorbance.

For a I: JOO soil:solution ratio and a 1 + 4 dilution

µg/ml --- ~ % Fe or Al

20


SC: DITHIONITE-CITRATE-EXTRACTABLE IRON AND ALUMINIUM

75

Dithionite-citrate reagent extracts 'free' (non-silicate) iron, which includes some inorganic crystalline forms, plus the fractions extracted by pyrophosphate and acid oxalate (organic complexes and short-range order inorganic forms).

The amounts of aluminium extracted by this reagent are not easy to interpret however, and usually are similar to the amount removed by acid oxalate. In some podzols and volcanic ash derived soils, acid oxalate extracts larger amounts of aluminium than does dithionite-citrate.

The method described is an adaptation of that of Holmgren (1967) and involves an overnight shaking at room temperature. This is more convenient than the traditional methods in which the sample is extracted a number of times, with heating.

PREPARATION OF REAGENTS SODIUM CITRATE, 22%. Dissolve 1100 g tri-sodium citrate (Na3C6H50,.2H20) in water and make to 5 litres. This solution should be made on the day required.

SODIUM DITHIONITE, Na2S20 4•

SUPERFLOC SOLUTION, 0.2%.

PREPARATION OF STANDARDS STOCK SOLUTION (!OOO µg Fe and Al/ml). See Method SA.

DITHIONITE-CITRA TE SOLUTION. Dissolve 55 g tri-sodium citrate and 5 g sodium dithionite in water and make to 250 ml.

WORKING STANDARDS. Pipette 0, 2, 5, 10, 15 and 20 ml of stock solution in 200-ml volumetric flasks; add 20 ml dithionite-citrate solution and make to volume with water. These solutions contain 0, 10, 25, 50, 75 and 100 µg Fe and Al/ml.

PROCEDURE Weigh I g soil (air-dry, < 0.25 mm) into a 250-ml centrifuge bottle.

Add I g sodium dithionite and 50 ml 22% sodium citrate solution.

Shake overnight (16 h) on an end-over-end shaker (about 50 r.p.m.).

Add 5 drops 0.2% superfloc. Shake vigorously for a few seconds, and filter through a no. 42 (Whatman) filter paper.

Dilute I + 19 with water and leave loosely stoppered for at least 2 days.


Read Fe and Al concentration by AAS.

NOTE: It has been found necessary to use AAS rather than FES for measuring Fe and Al levels in these extracts, because a significant background interference occurs. This presumably is caused by dissolved S02, and the amount will vary from sample to sample and also between samples and standards. Severe interference has also been noted when determining iron by AAS. This would probably be overcome by digesting samples on a water bath, but it has been found that leaving the samples loosely stoppered for at least 2 days is a convenient and effective means of avoiding the interference.

76

CALCULATION OF RESULTS Prepare standard curves for µg Fe or Al/ml against absorbance.

For a I :50 soil solution ratio and I + 19 dilution

µg/ml (Fe or Al) _ 0, F Al 10

-70 eor


REFERENCES Bl\LLANTYNE, A. K.; ANDERSON, D. W.; STONEHOUSE, H. B. 1980: Problems

associated with extracting Fe and Al from Saskatchewan soils by pyrophosphate and low speed centrifugation. Canadian Journal of Soil Science 60: 141-143.

DALY, B. K.; BINNIE, H.J. 1974: A leaching method for the extraction of acid oxalate soluble aluminium and iron from soil in conjunction with cation exchange leachings. Communications in Soil Science and Plant Analysis 5: 507-514.

HOLMGREN, G. G. C. 1967: A rapid citrate-dithionite extractable iron procedure. Soil Science Society of America Proceedings 31: 210-211.

McKEAGUE, J. A. 1967: An evaluation of 0.1 M pyrophosphate and pyrophosphatedithionite in comparison with oxalate as extractants of the accumulation products of podzols and some other soils. Canadian Journal of Soil Science 47: 95-99.

McKEAGUE, J. A.; DAY, J. H. 1966: Dithionite- and oxalate-extractable Fe and Al as aids in differentiating various classes of soils. Canadian Journal of Soil Science 46: 13-22.

PARFITT, R. L.; CHILDS, C. W. (In prep.): Estimation of forms of Fe and Al: a review and analysis of contrasting soils, using dissolution and Moessbauer methods.

SAUNDERS, W. M. H. 1965: Phosphate retention by New Zealand soils and its relationship to free sesquioxides, organic matter, and other soil properties. NZ Journal of Agricultural Research 8: 30-57.

SEARLE, P. L.; DALY, B. K. 1977: The determination of aluminium, iron, manganese and silicon in acid oxalate soil extracts by flame emission and atomic absorption spectrometry. Geoderma 19: 1-10.

TAMM, 0. 1922: Eine Methode Zur Bestimmung de anorganischen Komponente des Gelkomplexes im Boden. Meddelanden fran Statens skogsforsoksanstalt Stockholm 19: 387-404.

TAMM, 0. 1932: Uber die Oxalatmethode in der chemischen Bodenanalyse. Meddelanden fran Statens skogsforsoksanstalt Stockholm 27: 1-20.

77

9 : SOLUBLE SALTS

For chemical analyses of soils from humid regions, determinations of amounts of soluble salts present are usually confined to soils from areas affected by sea water or by abnormally high amounts of cyclic salts, and to soils which have received large dressings of fertilisers or animal excreta. Usually, the presence of salts will be suspected from knowledge of the nature of the sample, but the unexpected presence of salts is usually noted when % BS exceeds 100. If significant amounts of salts are present, tlie result for TEB,;m will be lower than TEB,"m·' because there is incomplete conversion of these cations to basic oxides during ignition. Low TEB,;m values are also obtained when ammonium acetate displaces adsorbed sulphate from strongly acid soils during leaching (Blakemore 1964), so it is necessary to differentiate between the effects of soluble salts and of displaceable (adsorbed) sulphate. This is usually done by carrying out a conductivity test on a water extract, the result of which will not only confirm or disprove the presence or absence of soluble salts but will also give an approximate measure of the amount present.

Methods for extraction of soluble salts, measurement of conductivity of the extracts and determination of the anions and cations present are described in detail by Melson (1956). The only significant changes which have been made to these methods are the use of atomic absorption spectrometry (AAS) or flame emission spectrometry (FES) for the determination of calcium, magnesium, potassium, and sodium, and the methods for the determination of sulphate ions. However, for the sake of completeness the conductivity method, determination of cations, and determination of the common anions (chloride and sulphate) are all described here.

78

9A :WATER EXTRACTION-1:5 SOIL:WATER RATIO

PREPARATION OF EXTRACT Shake 20 g soil (air-dry, < 2 mm) with 100 ml water for 30 min at about 20'C, and centrifuge. Alternative weights may be taken, provided a 1 :5 soil:water ratio is retained. For soils high in salts, centrifuging will provide an acceptable separation of extract (owing to flocculation of clay), but for less salty soils it may be necessary to filter through a retentive paper or a membrane filter.

Conductivity measurements should be made on the extract as soon as possible, whatever its state of clarity, because of possible changes in ionic content due to microbiological activity (Piper 1942).

79

9Al CONDUCTIVITY

PROCEDURE Measure the specific conductivity of the water extract in millimho (equals millisiemen), after first rinsing the cell with de-ionised wate·r.

Note the temperature.

CALCULATION OF RESULTS Correct the reading for cell constant and temperature (see table below) and express conductivity at 25°C as millimho/cm (K,,.)

K25• ~ reading (millimho) X cell constant X temperature correction factor (f,)

Temperature Correction Temperature Correction factor (f,) (°C) factor (f,) (°C)

8 1.499 21 1.092

10 1.421 22 1.067

12 1.350 23 1.044

14 1.284 24 1.021

15 1.254 25 I.OOO

16 1.224 26 0.979

17 1.196 28 0.941

18 1.168 30 0.906

19 1.142 32 0.873

20 1.118 34 0.843

From data of Whitney and Means, as given in Richards (1947)

The following equations give approximate values for total soluble salts in soils using 1:5 soil:water ratio:

K,,. (millimho/cm) X 0.35 ~ Total Soluble Salts (%)

and K,,. (millimho/cm) X 5 ~Total Soluble Salts (me./100 g)


80

9A2: WATER-SOLUBLE BASIC CATIONS (Ca, Mg, Kand Na) As with exchangeable-base analysis, either AAS or FES is suitable for determination of Ca, K and Na, but AAS is used for Mg. The standards used for exchangeable-base determinations are suitable for soluble salt analysis, providing the extracts contain approximately the same amounts of CsCl, HCl, ammonium acetate (CH3COONH4) and SrCl2 as the exchangeable-base standards.

PREPARATION OF REAGENT l + 9 DILUENT SOLUTION. See Method 6A2.

PREPARATION OF STANDARDS Although the proportions of cations present in soil/water extracts are usually different from the proportions in the standards used for exchangeable-base analysis (sodium and magnesium usually predominate in soil/water extracts), it has been found convenient and sufficiently accurate to use the exchangeable-base standards for determining concentrations of soluble salts.

PROCEDURE Dilute sample solutions, and water as a reagent blank, 1 + 9 with diluent solution.

Determine Ca, Mg, Na and K concentrations as for Method 6A2.

CALCULATION OF RESULTS Prepare standard curves of N X J0-4 against % transmission or absorbance.

For l: 5 soil:solution ratio and 1 + 9 dilution

N X2l0-

4 = me./100 g


81

9A3: WATER-SOLUBLE SULPHATE Two methods for the determination of sulphate in water extracts are described: 9A3.I, using the distillation method of Johnson and Nishita (1952) and 9A3.II, by turbidimetry, using an AutoAnalyzer.

9A3.I : Distillation

PREPARATION OF STANDARDS STOCK SULPHATE SOLUTION (100 µg S/ml). Weigh 0.5436 g K,so. (dried at 105°C) and dissolve in water. Make to 1 litre with water. Add a few drops of chloroform as preservative.

WORKING STANDARDS. Pipette 0, 2.5, 5, 10, 20, 30, 40 and 50-ml aliquots of stock solution (100 µg S/ml) into 100-ml volumetric flasks and make to volume with water. Add 2 or 3 drops of chloroform as a preservative. These solutions contain 0, 2.5, 5, 10, 20, 30, 40 and 50 µg S/ml. 2-ml aliquots will then contain 0, 5, 10, 20, 40, 60, 80 and 100 µg S.

PROCEDURE As for Method 11A2.I. Use 2-ml aliquots of the 1:5 soil:water extract, but if more than 50 µg SOJ--S ( > 1.56 me.%) is expected, dilute the extract before taking the 2-ml aliquot.

Use a 2-ml aliquot of water as a reagent blank.

CALCULATION OF RESULTS Construct a graph of µg S against absorbance and read off unknowns as µg S. as µg S.

Apply a blank correction in terms of µg S.

For a 1:5 soil:solution ratio and 2-ml aliquot

µg S X 0.0156 = SO./-(me./100 g)


9A3.II : Autoanalyzer

PREPARATION OF STANDARDS As for Method 9A3.I. If more than 10 µg SOJ--S (> 0.3 me.%) is expected, dilute the extract before pretreatment.

PROCEDURE As for Method 11A2.II.

CALCULATION OF RESULTS For a 1:5 soil:water extract

µg S/ml (in extract) 32

= so.r- (me./100 g)

82

9A4 WATER-SOLUBLE CHLORIDE This method is based on that ofSchales and Schales (1941). The extract is titrated with mercuric nitrate (Hg(N03) 2) and forms the very slightly dissociated mercuric chloride (HgCl2). The end point is recognised by the formation of f1 blue complex of the excess mercuric ions and the indicator, s-diphenylcarbazqne.

PREPARATION OF REAGENTS MERCURIC NITRATE SOLUTION (approximately 0.02 N). Add 20 ml 2 M

HN03, to approximately 700 ml water and then about 3.0 g of mercuric nitrate. Stir to dissolve, and make to I litre.

DIPHENYLCARBAZONE INDICATOR, 0.1%. Dissolve 0.1 g s-diphenylcarbazone in 100 ml 95% ethanol.. Store in a refrigerator.

CHLORIDE STANDARD, 0.02 N. Dry NaCl at 120°C for 4 h, dissolve 1.169 g in water, and make to I litre.

PROCEDURE Standardise the mercuric nitrate solution by titrating against 10 ml 0.02 N NaCl solution, using 10 drops of s-diphenylcarbazone indicator. The end point is recognised by the first appearance of a faint violet-blue colour.

Transfer a suitable aliquot of the extract to a conical flask and add 10 drops of indicator.

Titrate samples to the same end point with the standardised mercuric nitrate.

Titrate a similar aliquot of water as a reagent blank.

If a faint colour appears before the addition of mercuric nitrate, add a few drops of dilute HN03 (approximately 2 M).


CALCULATION OF RESULTS For a I :5 soil:water extract

Hg(N03) 2 (ml) X normality X 500 ~ CJ- (me./100 g)

aliquot taken (ml)

Use moisture factor (Method I) to correct to oven-dry basis.

REFERENCES BLAKEMORE, L. C. 1964: Effect of adsorbed sulphate on the determination of total

exchangeable bases in soils. NZ Journal of Agricultural Research 7: 435-438.

JOHNSON, C. M.; NISHITA, H. 1952: Microestimation ofsulfur in plant material, soils, and irrigation waters. Analytical Chemistry 24: 736-742.

METSON, A. J. 1956: Methods of chemical analysis for Soil Survey Samples. NZ Soil Bureau Bulletin 12: 208 p.

PIPER, C. S. 1942: 'Soils and Plant Analysis'. University of Adelaide, Adelaide. 368 p.

RICHARDS, L. A. (Ed.) 1947: Diagnosis and Improvement of Saline and Alkali Soils. U.S. Department of Agriculture Regional Salinity Laborat01y, Riverside, California. 140 p.

SCHALES, 0.; SCHALES, SELMA, S. 1941: A Simple and Accurate Method for the Determination of Chloride in Biological Fluids. Journal of Biological Chemistry 140: 879-884.

83

10: CALCIUM CARBONATE

If the pH ofa soil sample is more than 7.0, it should be analysed for carbonate, because the presence of calcium carbonate (CaC03) must be allowed for in determinations of exchangeable cations. Calcium carbonate occurs in soils from arid regions, soils derived from limestone, soils affected by lime-bearing ground waters, and heavily limed soils.

lOA : GASOMETRIC CARBON DIOXIDE METHOD

The method described below involves treating a known weight of soil with 20% perchloric acid and measuring the C02 evolved. The procedure is similar to that of Tinsley et al. (1951), as described in Metson (1956). However, the acid treatment is not carried out under vacuum and the C02 evolved is measured volumetrically instead of by titration (Searle 1967).

PREPARATION OF REAGENT PERCHLORIC ACID, 20% w/w. Dilute 1 part of 60% HCl04 with 2 parts of water.

PROCEDURE Weigh an appropriate amount of finely ground soil (air-dry, < 0.25 mm) into a 125-ml conical flask. (5 g covers the range 0-2.5% CaC03 and 1 g, 0-12.5% CaC03).

Fit the rubber stopper containing oxygen inlet, acid inlet, and gas outlet to the conical flask (Fig. 8).

Flush the flask with oxygen to remove atmospheric CO, and then close the gas inlet.

Add enough perchloric acid through the acid inlet to cover the soil, and gently shake the flask to ensure adequate mixing.

After about a minute, flush the flask with oxygen, transferring the gases (C02 + 0 2)

into the gasometric analyser where the percentage carbon value is read as in Method JA, using the 1-g scale. Because of the possibility of inefficient flushing or prolonged evolution of CO, from the sample, it is advisable to repeat the flushing and gasometric determination step until a zero reading is obtained.

The total of the individual readings is then corrected for temperature and pressure (see Method JA).

CALCULATION OF RESULTS 100

Total reading (corrected for temp. and press.) x ~ o/o CaC03 12 X sample wt

84

lOB DIFFERENTIAL METHOD

Although lack of sensitivity limits the accuracy of this method, especially when CaC03 contents are low ( < 5%), speed and simplicity makes it useful in assessing the CaC03 content of soils.

CaC03 is determined from the difference between total carbon % (Method 3A) and the carbon content of the sample after pretreatment with acid to remove carbonate carbon as carbon dioxide (Wimberley 1969).

PREPARATION OF REAGENTS HYDROCHLORIC ACID, I + 4.

IRON CHIPS. See Method 3A.

SILICIC ACID. See Method 3A.

PROCEDURE Weigh into an empty Leco combustion crucible the same weight of sample as used for total carbon method 3A.

Add I + 4 HCI drop by drop until all visible CO, evolution ceases. Samples with high carbonate content may require several additions of HCI.

Dry the crucible at I 00°C for 30 min.

Remove from oven, add one scoop of silicic acid (if weight of soil < I g) and then 2 scoops of iron chips.

Determine carbon content of the sample as for Method 3A.

CALCULATION OF RESULTS Total C % - acid-treated C % ~ carbonate C %

Assuming all carbonate to be CaC03, then:

100 Carbonate carbon X 12 ~ % CaC03

Oxygen from cylinder --controlled by needle valve

Oxygen inlet

20% w/w Perchloric Acid

reservoir

---::::::-- Stopcock

' /

- I- I- 1-1

I--~-

I

t I . -·-

Sample

- To gasometric carbon analyser (02 + C02)

Rub ber stopper

Gas outlet

Coni cal flask

Acid inlet

Figure 8 Apparatus for gasornetric CaC03 detennination

85

86

lOC WEIGHT-LOSS METHOD This method uses 1 + 1 HCI to evolve CO, from carbonate in the sample, the weight loss being recorded using an electronic top-pan balance. A correction for evaporation weight loss is applied to allow for the loss of water vapour and HCI. The HCl gas is lost from soil-catalysed decomposition of hydrochloric acid and therefore the weight-loss correction is obtained from the first sample and not from a beaker of acid.

The· method has only recently been devised and has not yet been rigorously tested, although good results were obtained using a small number of standard samples. Also recoveries of approximately 97% are obtained from added CaC03.

It is known that when appreciable quantities of magnesium carbonates are present, evolution of CO, is slow and approaches the evaporation constant.

Despite the lack of proving, and possible problems, the method is given because it provides a simple way of at least screening samples for CaC03 content.

PROCEDURE Weigh 5 g soil ( < 2 mm, air-dry) into a 200-ml tall-form beaker. More or less sample can be taken if the expected result is very low or very high.

Place approximately 40 ml 1 + 1 HCI into a 100-ml beaker.

Place both beakers on a top-pan balance (readable to 3 decimal places) and set to zero with tare adjustment.

Pour the acid into the 200-ml beaker containing the sample and place empty acid beaker back on balance pan. Start stop watch.

Every 2 or 3 min gently swirl beaker and place back on pan.

Note weight loss after 2 min and record at 1-min intervals for up to 10 min. By this time the weight loss should be constant and due only to evaporation (about 0.003-0.008 g/min).

Record total weight loss and number of minutes.

Subsequent samples need be left on the balance only until their rate of weight loss drops to the rate of weight loss by evaporation (usually 4 or 5 min).


1 () ( 1 I . . ) 100 100 0,1,CCO wt oss g - evap. wt oss mm X mm X -- X = ,o a 3 44 sample wt (g)

lOD CORRECTING AMMONIUM ACETATEEXCHANGEABLE Ca FOR CaC03-Ca

WHEN% CaCO, IS GREATER THAN 1

87

CaCO, at this level is not extracted quantitatively by 1 M ammonium acetate in the leaching process, and values obtained for exchangeable Ca, TEB";'"'' bases and % BS are incorrect and therefore are not quoted. The words 'free lime' are inserted under these headings on the analysis record.

WHEN % CaCO, IS LESS THAN 1

Where less than 1 % CaC03 occurs in New Zealand soils it is usually completely extracted by 1 M ammonium acetate during the leaching process and, for these soils, extractable calcium should be corrected using the relationship:

1 % Ca CO, ~ 20 me./ 100 g Ca

However, this relationship may not be valid for soils containing hard CaC03

fragments, such as shells, where complete solubility of the CaC03 is not obtained for levels of less than 1 %.

WHEN % CaCO, IS LESS THAN 0.1

Exchangeable Ca should not be corrected because the accuracy of the CaC03

determination at this level is inadequate for useful correction and the figures ' < 0.1 %' are entered under 'CaCO,' on the analysis card.

REFERENCES METSON, A. J. 1956: Methods of chemical analysis for soil survey samples. NZ Soil

Bureau Bulletin 12: 208 p.

SEARLE, P. L. 1967: Determination of carbon, calcium carbonate and sulphur in soil using high-frequency induction furnace equipment. NZ Soil News 5: 168-180.

TINSLEY, J.; TAYLOR, T. G.; MOORE, J. H. 1951: The determination of carbon dioxide derived from carbonates in agricultural and biological materials. Analyst, London, 76: 300-310.

WIMBERLY, J. W. 1969: A rapid method for the analysis of total and organic carbon in shale with a high-frequency combustion furnace. Ana/ytica Chimica Acta 48: 419-423.

89

11: SULPHUR

llA EXTRACTABLE SULPHUR

Ensminger (1954) showed the presence of soil sulphate which is insoluble in water but replaceable by other strongly adsorbed anions such as hydroxyl, acetate, phosphate and fluoride. This sulphate was termed 'adsorbed sulphate', and accumulations of it were found particularly in subsoils containing clays high in aluminium, iron oxides and 1:1 (kaolinitic) clay minerals. Ensminger measured this sulphate turbidimetrically as BaS04• Since then it has generally been accepted that although smaller amounts of adsorbed sulphate are usually found in topsails, measurement of it affords a good guide to available sulphur status, and replacement by phosphate solutions is now widely used to measure available sulphur status.

Johnson and Nishita (1952) developed a very sensitive distillation method for measurement of sulphate. In this method sulphate is reduced to H2S, which is converted to methylene blue and measured colorimetrically. These authors noted that as well as sulphate, sulphides, sulphites, and possibly some very labile forms of organic sulphur, would be included. Fot several reasons (e.g., high sensitivity) the Johnson and Nishita method became widely used to measure sulphur in soil extracts, and a looseness of terminology crept in. Both 'adsorbed sulphate' and 'adsorbed sulphur' have been commonly used, even though the Johnson and Nishita method is known to include some organic forms of sulphur.

To measure sulphate-sulphur in the extracts, charcoal pretreatment to remove organic sulphur, is carried out before Johnson and Nishita distillation. AutoAnalyzer turbidimetric methods (which only measure sulphate) also need charcoal pretreatment to remove organic matter which interferes with the precipitation reaction.

The following fractions can be determined:

Phosphate-extractable sulphur: as determined by Johnson and Nishita distillation or by AutoAnalyzer turbidimetry on pre-oxidised extracts.

Phosphate-extractable sulphate: as determined on charcoal-treated extracts by Johnson and Nishita distillation or by AutoAnalyzer turbidimetry.

Adsorbed sulphate: calculated by subtracting H20-soluble sulphate (Method 9A3) from phosphate-extractable sulphate.

The I-hour extraction time in the method described below is shorter than that in the method in the previous edition ( 1981 ). This minimises the extraction of organic compounds but is still adequate to remove sulphate that is important in terms of plant nutrition (Lee et al. 1981).

90

llAl PHOSPHATE-EXTRACTABLE SULPHUR

PREPARATION OF REAGENTS EXTRACTING SOLUTION, 0.04 M Ca(H,PO,), (Searle 1979). Add 40 g H3PO,, cone. (88%), to about 2.5 litres water in a 5-litre flask. Slowly add with stirring a suspension containing 20 g CaC03 in about a litre of water. Dilute to 4.5 litres with water. Adjust to pH 4 by adding cone. H3P04 drop by drop whilst stirring. If the pH goes below 4, adjust with calcium hydroxide (made by shaking CaO with water and filtering). Make to 5 litres with water.

REDUCTION MIXTURE. Mix together:

either 500 ml hydriodic acid, sp. gr. I. 7, 125 ml hypophosphorous acid, 50% w/w, and 200 ml formic acid, 90%.

or 500 ml hydriodic acid, sp. gr. 1.94, I 71 ml hypophosphorous acid, 50% w /w, 274 ml formic acid, 90%, and 185 ml water.

Heat the mixture to 115-11 re for I 0 min, bubbling nitrogen through it to remove any hydrogen sulphide evolved, and to prevent bumping.

The reduction mixture may be regenerated, as follows:

to 200 ml used mixture, add: 20 ml hydriodic acid (sp. gr. I. 7 or 1.94) 4 ml hypophosphorus acid, 50% w /w, and 36 ml formic acid, 90%.

Heat, with nitrogen bubbling through to remove water, until the temperature reaches 115-11 re, and then reflux (with nitrogen bubbling through) for 1-2 h. Johnson and Nishita (1952) reported that the mixture may be regenerated as many as four times provided excessive amounts ofperchloric acid have not been ~ntroduced with the samples.

SULPHIDE-ABSORBING SOLUTION (zinc acetate-sodium acetate). Weigh out 50 g zinc acetate and 12.5 g sodium acetate, hydrated, and dissolve in water. Filter through a no: 541 (Whatman) filter paper and make up to I litre with water.

COLOUR REAGENT ('amino reagent', Gustafsson 1960). This reagent is prepared by acidifying the amino compound, NN'-dimethyl-p-phenylenediamine sulphate (Eastman Cat. No. 1333) with H,SO, as follows:

Dissolve 2.0 g of the amino compound in 1500 ml water, add 400 ml H2S04,

cone., and dilute to 2 litres with water.

AMMONIUM FERRIC SULPHATE SOLUTION. To 50 g ammonium ferric sulphate, (NH4),SO,.Fe2(S04) 3.24H20), add 10 ml H,S04, cone., and 390 ml water. Filter through a no. 42 (Whatman) filter paper.

PREPARATION OF STANDARDS STOCK SULPHATE SOLUTION (100 µg S/ml). Weigh 0.5436 g K,SO, (dried at I 05°C) and dissolve in 0.04 M Ca(H2P04) 2 extracting solution. Make to I litre with the extracting solution. Add a few drops of chloroform as preservative.

WORKING STANDARDS. Pipette 0, 2.5, 5, 10, 20, 30, 40 and 50-ml aliquots of stock solution (100 µg S/ml) into 100-ml volumetric flasks and make to volume with extracting solution. Add 2 or 3 drops of chloroform as a preservative. These solutions contain 0, 2.5, 5, 10, 20, 30, 40 and 50 µg S/ml. 2-ml aliquots will then contain 0, 5, 10, 20, 40, 60, 80 and 100 µg S.

91

PROCEDURE PREPARATION OF EXTRACTS. Weigh 5 g soil (air-dried, < 2 mm) into stoppered 50-ml polypropylene centrifuge tubes and add 25 ml of extracting solution.


Shake on an end-over-end shaker (about 50 r.p.m.) for 1 h at 20°C and then centrifuge at 2000 r.p.m. for 15 min.

DISTILLATION AND DETERMINATION. The distillation apparatus used at NZ Soil Bureau (Fig. 9) is a modification of that described by Johnson and Nishi ta ( 19 52). A bank of 6 individual distillation units forms a convenient group for simultaneous operation.

At the beginning of a set of determinations, flush out the apparatus with copious quantities of water (using a vacuum to suck the water through) and suck air through the apparatus to remove the water.

With a filter paper, remove any water adhering to the inner surface of the il14 cone.

Turn on nitrogen supply.

Wash carrier gas attachment and dry thoroughly with filter paper. Connect to apparatus.

Add water to the gas washer (about 1/J full) and connect this to the apparatus.

Fit a washed glass delivery tube to the rubber on the side arm of the gas washer.

Prepare collecting solution by measuring out 70 ml distilled water into a 100-ml volumetric flask and pipetting into this 10 ml zinc acetate-sodium acetate reagent.

Fit the collecting flask to the distillation apparatus so that the glass delivery tube nearly reaches to the bottom.

Transfer 2-ml aliquots of sample extracts and standards to 50-ml flask (Fig. 9).

Add 6 ml reduction mixture to the flask and connect the flask to the apparatus.

Turn on the water supply to the condensers and adjust the nitrogen flow so that the rate through each unit is about 150 ml/min.

Turn on heat source and carry out the distillation for 20 min.

Disconnect the delivery tubes and collecting flasks from the apparatus, leaving the delivery tube inside the flask.

Stopper the collecting flask and set aside for colour development.

Turn off the heat source.

Disconnect the digestion flasks and retain the contents for regeneration of the reduction mixture.

Fit another delivery tube and the next receiving flask containing zinc acetatesodium acetate reagent and distilled water. Carry out the distillation on this and subsequent runs as for the first run. Change the water in the gas washer after 6 runs.

When all the distillations have been completed and the contents of the receiving flasks have been equilibrated with room temperature, carry out colour development and measurement as follows:

Add 10 ml colour reagent (using a rapid-delivery safety pipette, or an automatic dispensing pipette), re-stopper and shake gently.

Add 2 ml ammonium ferric sulphate reagent; re-stopper flask and shake vigorously. Rinse the glass delivery tube into the flask and remove.

Make up the contents of the flask to the 100-ml mark with water.

92

H.o out ... 1-----c:::::::

Condenser --------..J

H.O in------c:::::.J

814 joint ---------1

814 joint

J--tt------Splash head

t+------ Rubber joining tube r-----ftHc----- Gas washer

'--.!l..-/--- 100 ml volumetric flask

N~rogen carrier ~::::;:::::=::=:P -----------Nitrogen inlet

gas attachment ---------4

814 joint --------1

Electric heating mantle .or gas-heated g raph~e block

Figure 9 Modified Johnson and Nishita apparatus

After at least 30 min read absorbance at 670 nm on a suitable spectrophotometer (the colours are reported to be stable for several days if stored in the dark).

The absorbance maximum is rather sharp at 670 nm (Johnsori and Nishita 1952) and should be tested for the particular spectrophotometer in use.

CALCULATION OF RESULTS Construct a graph of µg S against absorbance and read off unknowns as µg S.

For a 1:5 soil:solution ratio and a 2-ml aliquot

µg S X 2.5 ~ phosphate-extractable sulphur in soil (µg S/g)


93

94

11A2 PHOSPHATE-EXTRACTABLE SULPHATE

PREPARATION OF REAGENTS ACTIVATED CHARCOAL (Sigma Chemical Co. Cat. No. C-3486). Boil 500 g with about 500 ml of 1 + 1 HCI for 10-15 min. Allow to settle and decant the bulk of the supernatant. Transfer charcoal to a large Buchner funnel and wash thoroughly with deionised water. Dry in oven at 105°C.

PROCEDURE Prepare extract as described in llAl.

Pour 15 ml of extracts, blanks and standards into calibrated 25-ml test tubes.

Add 1 calibrated scoop of charcoal and 1 ml of 1 M HCI from dispenser, stopper, and allow to stand for 30 min, shaking occasionally.

Add 1 further scoop of charcoal. Shake, and allow to settle overnight.

Filter through no. 42 (Whatman) paper and collect filtrate. If Method l 1A2.II is to be used, filter directly into sample cups, discarding the first few ml.

11A2.I Distillation Measure S in extracts using reagents, standards and distillation procedure as described In Method llAI.


Construct a graph of µg S against absorbance and read off unknowns as µg S.

For a 1:5 soil:solution ratio and a 2-ml aliquot

µg S X 2.5 = phosphate-extractable sulphate-sulphur in soil (µg S/g)


11A2.II : AutoAnalyzer The AutoAnalyzer 1)1ethod is based on the turbidimetric measurement of the amount of barium sulphate precipitate formed when the soil extract is mixed with barium chloride in acid conditions. Gelatine is added to keep the precipitate in suspension. Charcoal pretreatment decolorises the soil extract, thus removing organic compounds that may interfere with the turbidimetric measurement or inhibit the precipitation.

PREPARATION OF REAGENTS BARIUM CHLORIDE REAGENT. Dissolve 1.0 g gelatine sheet as follows: cut sheet into small pieces and weigh. Place gelatine in a I-litre beaker and add 50 ml cold water. Boil 200 ml water and pour over soaking gelatine, stirring to dissolve. Make up to 800 ml with cold water. Add 25 g BaCl,.2H,O to the cold solution. Dissolve, filter and make up to 1 litre with water. Make fresh before use.

SULPHATE SEED SOLUTION (4 µg S/ml). Add 50 ml HCI cone. to 800 ml water. Add 40 ml of 100 µg S/ml stock solution and make to 1 litre with water. Add 5 drops Brij 35 and mix. Higher concentrations of S in the seed solution may be required, depending on the purity of the charcoal used.

WASH SOLUTION FOR BARIUM CHLORIDE LINE. Add 50 ml HCI cone. to 1 litre of water. Add 5 drops Brij 35 and mix.

WASH SOLUTION FOR SAMPLE LINE. Water plus 5 drops Brij 35/L.

m 10 turn , 5 turn ixing coil mixing coil

l ...... ". '.". ... I

waste

colorimeter 630 nm

() waste

95

PREPARATION OF STANDARDS STOCK SOLUTION (100 µg S/ml). Weigh 0.5436 g K,SO, (dried at 105°C) and dissolve in 0.04 M Ca(H,P0,)2 extracting solution. Make to 1 litre with 0.04 M

Ca(H2P04) 2 extracting solution and add a few drops of chloroform as preservative.

WORKING STANDARDS. Dilute 0, 5, 10, 15 and 20-ml aliquots of stock solution (JOO µg S/ml) to 200 ml with 0.04 M Ca(H2P04) 2 extracting solution. These standards then contain 0, 2.5, 5.0, 7.5 and 10.0 µg S/ml.

PROCEDURE Set up AutoAnalyzer as shown in manifold diagram (Fig. 10) with 630 nm filters in the colorimeter and the water bath off. Place the sample probes in the first hole of the sampler arm (the one normally used) and the probe for the BaCl2 in the second hole so that the tip of the probe is about 2 cm below the surface of the HCI wash solution. Position the detachable holder containing the BaCl2

solution onto the outside of the sampler cover so that in the sample mode the probes dip into the sample and BaCl2 solutions respectively.

Pump reagents and wash solutions through the system for about 30 minutes to ensure complete flushing of the analytical system and wash receptacles.

Set up the recorder baseline with the colorimeter baseline control.

Sample top standard (10 µg S/ml) and adjust to about 80% of recorder full scale with the standard calibration control. Readjust baseline if necessary.

Sample standards and unknowns at a rate of 20/hr with a 1:4 sample:wash ratio.

I

sam pie wash. receptacle

BaC I, wash receptacle

Flow Rate ml/min

0.32 air

Reagent

1.00 sam pie probe

0.32 sulphate seed solution

0.6 BaCI, probe

1.4 water wash

1.2 HCI wash

Tube Colour Code

black

grey

black

white

blue/yellow

yellow

0.6 pull from colorimeter white

1.2 BaCI, reagent yellow

1.4 pull from BaC I, blue/yellow receptacle

Figure 10 Flow diagram of AutoAnalyzer 1nanifold for sulphate determination

96

CALCULATION OF RESULTS Prepare a standard curve of µg S/ml against peak height. ,

For a 1 :5 soil:solution ratio

µg S/ml X 5 = phosphate-extractable sulphate-sulphur (µg S/g)


REFERENCES ENSMINGER, L. E. 1954: Some factors affecting the adsorption of sulphate by Alabama

soils. Soil Science Society of America Proceedings 18: 259-264.

GUSTAFSSON, LILY 1960: Determination of ultramicroamounts of sulphate as methylene blue. Part II. The reduction. Talanta 4: 236-243.

JOHNSON, C. M.; NISBIT A, H. 1952: Microestimation of sulfur in plant material, soil, and irrigation waters. Analytical Chemistry 24: 736-742.

LEE, R.; BLAKEMORE, L. C.; GIBSON, E. J.; DALY, B. K. 1981: Effect of extraction time and charcoal treatment on the adsorbed sulphate values of several New Z.ealand topsoils. Communications in Soil Science Plant Analysis 12: 1195-1206.

SEARLE, P. L. 1979: Measurement of adsorbed sulphate in soils-effect of varying soilextractant ratios and methods of measurement. Ne1v Zealand Journal of Agricultural Research 22: 287-290.

97

12 : ANALYSES OF PLANT LITTERS AND PEATS

In the analytical work associated with soil surveys a sample should be treated as a litter when the loss on ignition exceeds 80%. Peat samples are treated in a similar way to litters. The sample should be air-dried and pulverised in a Wiley mill, or similar apparatus, to obtain a homogeneous sample. The methods are basically those outlined by Metson (1956) but have been updated for modern instrumentation.

The following methods are used on these materials:

Loss on ignition, acid-insoluble ash, total phosphorus, total bases. pH in H20 (Method 2A), carbon, (Method 3A), nitrogen (Method 4B) and moisture factor (Method 1) are also measured.

12A LOSS ON IGNITION

PROCEDURE Weigh 10.00 g of air-dry material into a silica basin that has been ignited and weighed.

Heat on a burner until the material has completely charred, but do not allow it to catch fire (smother flames with watch glass if necessary).

Ignite at 500°C in a muflle furnace until stirring with a fine wire shows that all organic matter has disappeared (i.e., no glowing particles remaining).

Cool in a desiccator and reweigh.

Keep the ash for analysis.

CALCULATION OF RESULTS Weight of ash ~ weight of basin + ash - weight of basin

For a 10-g sample

weight of ash X 10 ~%ash

100 - (% ash X moisture factor) ~ % loss on ignition

Carry out determinations of acid-insoluble ash (if required), total phosphorus and total bases only if the loss on ignition exceeds 80%. If not, samples are treated as soils and analysed by the methods already described.

98

12B: PREPARATION OF SOLUTION FOR ANALYSES AND DETERMINATION OF ACID-INSOLUBLE ASH

PR()CEDURE To the ash from the loss on ignition determination add 20 ml 1 + 4 HCl, and about 20 ml water.

Carry a reagen·t blank throughout the following procedures for total phosphorus and total bases determinations.

Cover with a watch glass and digest for 30 min on a water bath at about 100°C.

Filter through no. 542 (Whatman) filter paper into a 250-ml volumetric flask and wash residue with hot water.

Make filtrate to 250 ml with water for total phosphorus and total bases determinations.

If measurement of acid-insoluble ash is required, place filter paper and residue in a silica crucible that has been ignited and weighed and char slowly over a burner.

After charring, heat in a muffie furnace at 500°C for 30 min.

Cool in a desiccator and reweigh.

CALCULATION OF RESULTS For a 10-g sample

weight of residue (g) X 10 = % insoluble ash


99

12C TOTAL PHOSPHORUS

PROCEDURE Determine phosphorus (µg/ml in extract) by either Method 5D.I or 5D.IL If Method 5D.II is used, dilute an aliquot from the acid-insoluble ash filtrate 1 + 4 with 0.55 M H,S04 (31 ml cone. H,SO,/litre).

CALCULATION OF RESULTS If Method 5D.I is used

for a 10-g sample made to 250 ml and for a 2-ml aliquot

µg P in final 100 ml X 1.25 ~ Total P (mg/100 g)

If Method 5D.II is used

for a 10-g sample made to 250 ml and for a 1 + 4 dilution

µg P/ml X 12.5 ~ Total P (mg/JOO g)

Apply a blank correction, then use moisture factor (Method 1) to correct to oven-· dry basis.

100

12D TOTAL BASES (Ca, Mg, Kand Na)

PROCEDURE Dilute an aliquot of the insoluble ash filtrate 1 + 9 using the 1 + 9 bases diluent (see Method 6A2).

Carry out the determination of individual bases, using the standards and procedure as outlined in Method 6A2.

CALCULATION OF RESULTS For a 10-g sample made to 250 ml and diluted 1 + 9

(N X 10-•) X 2.5 ~ (Ca, Mg, Kor Na) (me.jlOOg)


REFERENCE METSON, A. J. 1956: Methods of chemical analyses for soil survey samples. NZ Soil

Bureau Bulletin 12: 208 p.

IOI

13. : REPORTING AND RATING OF RESULTS

Generally, results should be reported so that the uncertainty is restricted to the last figure given. Results should be rounded off to the recommended number of figures after the moisture factor has been applied. On this basis the following should be used as a guide to the number of significant decimal places reported.

Property determined

pH

C % >I

N%

C/N

<l

Available P (µg/g)

0.5 M H,S04-soluble P (mg/100 g)

Organic P (mg/ 100 g)

Total P (mg/100 g)

P retention (%)

CEC (pH 7), ECEC, CEC (pH 8.2) (me.jlOO g)

TEB,;," (me./100 g)

:!: bases (me./ 100 g) > 10 <10

BS (%)

Exchangeable Ca, Mg, K, Na (me.f!OO g) > 10 < 10

1 M KCI-extractable Al (me./ 100 g)

Exchangeable acidity (me./100 g)

K (me.%) >I <l

Mg, (me.%) > 10 < 10

Acid oxalate Al, Fe and Si (%) > 1 <I

Pyrophosphate-extractable Al, Fe and C

Citrate-dithionite Al and Fe

Phosphate-extractable sulphur or sulphate (µg/g)

Water-soluble cations and anions . . . . . .

Conductivity K25• (millimho/cm) . . . . . .

Total soluble salts (%) (from conductivity)

Calcium carbonate (%) < 0.1 <I > 1

Recommendation

I decimal place

1 decimal place 2 decimal places

2 decimal places

whole number

whole number

whole number

whole number

whole number

whole number

1 decimal place

1 decimal place


whole number


1 decimal place

1 decimal place

l decimal place 2 decimal places

whole number I decimal place


as for acid oxalate

as for acid oxalate

whole number

as for exchangeable bases

2 decimal places

2 decimal places

quote as <0.1% 1 decimal place whole number

102

RATINGS FOR CHEMICAL PROPERTIES The ratings given in the following table are intended as a guide to the interpretation of the chemical analyses made on 'type samples' taken during soil survey work in New Zealand. These ratings include those set out by Metson (1956) together with those subsequently added in order to cover the present-day range of chemical analyses being carried out.

It is important to realise that the ratings have been devised to indicate the ranges of values encountered in analyses of New Zealand soils by the methods described. There has not been a strong agricultural basis for them and, although some deficiency levels have been taken into account, their use for making fertiliser recommendations is not advisable.

It is also important to realise that the ratings, although originally devised for topsoils, are now used to describe all parts of the soil profile. New Zealand soils display considerable nutrient enrichment of their topsoils due to the effects of the active organic cycle under forest and permanent pasture systems. As a consequence, the ranges of the ratings are often much larger than would be necessary to describe subsoils adequately.

RATINGS FOR CHEMICAL PROPERTIES The following ratings of chemical properties are used by NZ Soil Bureau for New Zealand soils.

Organic matter Phosphorus Organic Total 0.SM Inorg-

Rating pH (1:2.5 soil:water) c N C/N Truog Olsen H 2S04 anic Org-anic Total

--(%)-- -(µg/g)- --(mg/100 g)--(IA)I (3A or B) (4A or B) (SA) (SB) (SD) (5E) (5F)

>9.0 (extremely alkaline)

103

p retn. (%) (50)

Very high 8.4-9.0 (strongly alkaline) >20 > 1.0 >24 > 50 > 50 >40 > 50 > 70 > 120 90-100 7.6-8.3 (moderately alkaline)

High 7.1-7.5 (slightly alkaline) 10-20 0.6-1.0 16-24 30-50 30-50 20-40 30-50 50-70 80-120 60-90 6.6-7.0 (near neutral)

Medium 6.0-6.5 (slightly acid) 4-10 0.3-0.6 12-16 20-30 20-30 10-20 20-30 20-50 40-80 30-60 5.3-5.9 (moderately acid)

Low 4.5-5.2 (strongly acid) 2-4 0.1-0.3 10-12 10-20 10-20 5-10 10-20 10-20 20-40 10-30

Very low <4.5 (extremely acid) <2 < 0.1 < 10 < 10 < 10 <5 < 10 < 10 <20 0-10

Exchange Cation exchange properties (NH40Ac, pH7) KCI-extr. Acidity Reserve

Rating CEC ~Bases BS Ca Mg K Na AI (pH 8.2) K, Mg, - (me./100 g) - (%) (me./100 g) (me./100 g)

(6A4) (6A3) (6A5) (6A2) (6A2) (6A2) (6A2) (681) (6CI) (7A) (78)

Very high >40 > 25 80-100 >20 >7 > 1.2 >2 >5 >60 > 0.5 > 30

High 25-40 15-25 60-80 10-20 3-7 0.8-1.2 0.7-2 2-5 30-60 0.35-0.5 15-30

Medium 12-25 7-15 40-60 5-10 1-3 0.5-0.8 0.3-0.7 0.5-2.0 15-30 0.20-0.35 7-15

Low 6-12 3-7 20-40 2-5 0.5-1 0.3-0.5 0.1-0.3 0.1-0.5 5-15 0.10-0.20 3-7

Very low <6 <3 <20 <2 <0.5 <0.3 <0.1 < 0.1 <5 <0.10 <3

Dithionate- Phosphate-Pyrophosphate- citrate- extractable

Acid oxalate-extractable extractable extractable sulphur or Soluble salts Rating AI Fe Si AI Fe Al Fe sulphate Conductivity2 o/o salts

(%) (%) (%) (%) (%) (%) (%) (µg S/g) (millimho/cm) (SA) (BA) (SA) (8B) (SB) (SC) (SC) (llA or B) (9A) (9A)

Very high > 3.0 > 2.0 >2.0 > 1.2 > 2.0 >4.0 > 150 >2 >0.7

High 1.0-3.0 1.0-2.0 > 0.5 0.8-2.0 0.6-1.2 1.0-2.0 2.0-4.0 50-150 0.8-2 0.3-0.7

Medium 0.5-1.0 0.5-1.0 0.15-0.5 0.4-0.8 0.3-0.6 0.5-1.0 1.0-2.0 15-50 0.4-0.8 0.15-0.3

Low 0.2-0.5 0.2-0.5 0.05-0.15 0.1-0.4 0.1-0.3 0.2-0.5 0.5-1.0 5-15 0.15-0.4 0.05-0.15

Very low <0.2 <0.2 <0.05 <0.1 <0.l <0.2 < 0.5 <5 <0.15 <0.05

1 Numbers in brackets refer to the number of the analytical method in NZ Soil Bureau Scientific Report 80. 2 Conductivity of the 1 :5 extract at 25°C

Documents

Methods for chemical analysis of soils