Soil Chemistry Chapter 5. Soil Analysis Ch52 5.1 Introduction basic chemical composition of a soil is less useful than a knowledge of its component

Soil Chemistry

Chapter 5

Soil Analysis Ch5 2

5.1 Introduction

basic chemical composition of a soil is less useful than a knowledge of its component minerals and organic materials.

these dictate: reactions that occur in the soil availability of nutrients

Soil Analysis Ch5 3

Exercise 5.1

Decrease uptake by plants leaching conversion into

insoluble forms

Increase addition of fertiliser decomposition of plants animal poo dissolving of rock

Soil Analysis Ch5 4

5.2 Clay Minerals

naturally occurring inorganic compounds form initially in the crystallisation of molten rock

material known as primary minerals

eg olivine, quartz, feldspar and hornblende not stable when exposed to water, wind and

extremes of temperature break down physically and chemically reform and crystallise in a different structure

Soil Analysis Ch5 5

Clay minerals called secondary minerals

eg vermiculite, montmorillonite and kaolinite tend to be much smaller in particle size than primary

minerals most commonly found in the clay fraction of soils only the youngest and unweathered of soils will not

contain mainly secondary minerals

Soil Analysis Ch5 6

The Earth’s crust

Al

Ca

Fe

K

Mg

Na

O

Others

Si

Soil Analysis Ch5 7

oxygen is negatively charged the other major elements are positively charged oxygen bonds with one or more of the cations,

producing a chemistry of oxides silicon oxides (silicates) aluminium oxides (aluminates)

generally in combination as aluminosilicates these dominate the minerals low levels of other elements account for the

differences in minerals

Soil Analysis Ch5 8

Si binds to four oxygens in a tetrahedron Al has six oxygens (often as OH) in an octahedron not a matter of individual SiO4 or Al(OH)6 units some Os are shared between the silicate or aluminate

units most common structure in clay minerals is the

formation of sheets “flat” layers of silicate tetrahedra or aluminate

octahedra these sheets stack on top of each other held together by hydrogen bonding or electrostatic

attraction

Soil Analysis Ch5 9

Common sheet arrangement in clay minerals (tetrahedrons in grey)

1:1 2:1 2:2

Soil Analysis Ch5 10

real clay crystals are not pure silicates or aluminates some Si or Al atoms are substituted during the

crystallisation process creates spare charges which give the overall crystal a

charge balanced by loose cations or anions

Si

O

O

OOSi replaced

by Al in crystalAl

O

O

O O-

X+

O has only 1 bond,so has -ve charge;requires balancingpositive charge from free cation


these cations generally are held on the surface of the clay are not strongly held can be exchanged for other cations in an equilibrium

process measured as the cation exchange capacity (CEC) soil pH has no effect on the exchange capacity from the

clay minerals


as minerals weather, they lose silicon this leads to increasing proportions of aluminate

in weathered clays Al-OH species are amphiprotic soils dominated by oxides of aluminium (and

other metals) can have positive sites in acidic soils

this allows anion exchange

Al-OH + H+ <=> Al-OH2+ + X-


5.3 Ion exchange in soils

when the loosely held cations or anions on the mineral surfaces are replaced by ions of the same charge (sign and magnitude) in solution

cation exchange is by far the most common necessary for soil fertility as soils weather, they lose cation exchange capacity

and lose fertility


Cation Exchange clay minerals have negative charge due to substitution of

aluminium or silicon in the crystal lattice humus also contributes negative charge, due to the

presence of dissociated organic acids humus-COOH humus-COO- + H+

Exercise 5.2 What effect would soil pH have on the amount of

cation sites from humus?

low pH, less dissociated acid, less sites


a cation in solution replaces an adsorbed cation on the soil particle

eg soil-Na + K+ (aq) soil-K + Na+ (aq)

charges that are balanced, not number of charged species.

Class Exercise 5.3 Write an equation for the exchange of adsorbed sodium

with solution calcium.

soil-Na + soil-Na + Ca2+ (aq) soil=Ca + 2Na+ (aq)


exchange is equilibrium reversible and dependent on the levels of each of the

species, particularly the solution species eg if a soil solution becomes depleted in calcium,

then some calcium will desorb from an exchange site into solution

known as buffering in all but the most leached and infertile of soils, there

will be a balance between adsorbed and dissolved ions


Exercise 5.4 What do you think would happen to a soil which is

treated with lime (calcium hydroxide), in addition to a pH change?

high concentration of Ca in solution this would be partly reduced by exchange with the

soil cations


Cation exchange capacity (CEC)

the moles of exchangeable positive charge per unit mass 100 g of dry soil

usually mmole/100g or cmole/kg (the same value)

Ca & Mg contribute twice as much to the CEC as an equivalent number of sodium and potassium ions because of their 2+ charges


Class Exercise 5.5 Comment on the trend in

CEC in Table 5.1.

CEC increases with higher clay levels

Soil CEC

Sand 2-4

Sandy loam 2-12

Loam 7-16

Silt loam 9-26

Clay, clay loam

4-60


Significance of CEC uptake of nutrient ions from plant roots occurs from

solution only as cations are absorbed into the roots, they are

replaced in the soil solution by H+ ions when the exchange equilibrium is disturbed, some of

that ion will desorb from the soil particles replaced by another ion if the nutrient is a weakly adsorbed one, such as K,

there may not be enough adsorbed to replenish the soil, presenting a fertility problem

K is the most likely cation to be in short supply


Anion exchange the important soil anions, nitrate and phosphate,

behave very different at exchange sites nitrate and chloride are only weakly held at positive

sites more likely to be found in soil solution phosphate and sulfate are very strongly bound to the

exchange sites phosphate can become covalently and irreversibly

bound


Soil pH one of its most important properties it affects so many other soil properties, (eg ion

exchange and nutrient availability) soil pH comes about from a balance between acidic

and alkaline species reflects mainly the levels of dissolved H+ and OH-,

but also the adsorbed H+ on cation exchange sites normally ranges from 4-9


Sources of soil acidity rain - polluted or fresh will be slightly acidic due to

dissolved gases

microbial and root respiration – this produces CO2, which is slightly acidic in solution

oxidation of organic matter – this produces organic acids known as humic acids, together with nitric and sulfuric acids


Sources of soil alkalinity

carbonate minerals – calcium and magnesium carbonate are common materials in minerals

they are slightly soluble in water, and produce OH- as they dissolve

these cations and Na & K are known as bases because of their association with alkaline soils

mineral weathering – many primary minerals as they weather release hydroxide salts of the basic cations


Trends in soil pH as soils age by weathering and leaching, they tend to

become more acidic primary minerals that release alkaline materials are

replaced by neutral or slightly acidic secondary minerals

leaching removes the carbonate minerals weathering occurs from the surface downwards so

that the A and B horizons will tend to be more acidic than the C horizon


Significance of soil pH nutrient availability – the ability of plants to take up

nutrients is very much dependent on the soil pH


Significance of soil pH effect on soil organisms – soil organisms prefer

different pH levels acid-sulfate soils - soils that are rich in inorganic

sulfide minerals, such as pyrites, can lead to the formation of excessive levels of

sulfuric acid through oxidation soil pH dives to very low levels causes solubilisation of toxic levels of aluminium,

manganese and iron from soil minerals plant preferences – most alkaline soils; a few which

need acidic soils


Soil pH management soils tend towards lower pH values as they age the main need for pH management is to making the

soil more alkaline most common method by liming agricultural lime is a mixture dominated by CaCO3,

but also containing MgCO3 and Ca(OH)2 comes from ground limestone, add the nutrients calcium and magnesium to the soil dolomite lime has a higher proportion of magnesium

carbonate to reduce pH , add Fe, S or peat


Exercise 5.9 What factors will affect the amount of liming

required?

buffering capacity pH


Redox potential (Eh)

a measure of its ability to produce oxidation or reduction of chemical species in it

the most important soil property indicated by the soil Eh is whether it is aerobic or anaerobic

aerobic soils give a positive value the lower the value the more anaerobic the conditions a value that is affected by soil pH

Documents

Soil Chemistry Chapter 5. Soil Analysis Ch52 5.1 Introduction basic chemical composition of a soil is less useful than a knowledge of its component