RELATION OF SOIL ACIDITY TO SOIL CLASSIFICATION.

By P. L. Gile,1 U. S, Bureau of Soils.

It is becoming a custom of scientific journals as well as of popularmagazines to summarize the story under the title, so it may be good form todisclose the plot of this article at once, viz: soil acidity has no directrelation to soil classification.

There are still wide differences of opinion concerning the manner inwhich soil acidity affects plants. The more recent view is that the acidityitself is not injurious, but the substances which accompany acidity in thesoil solution, especially aluminum, ^re injurious. This theory, however,does not satisfactorily explain the beneficial effect of acidity for othercrops. My opinion is that we may well find soil acidity limits crop growthin determining, under many conditions, the quantities of certain essentialnutrients in the soil solution, probably iron and phosphoric acid in par-ticular.

In spite of the fact that the action of soil acidity on the crop hasnot been satisfactorily, or at least fully, explained and in spite of thefact that methods of measuring soil acidity are also unsettled, an immenseamount of practical good has developed from recognition of acidity as a con-trolling factor in soil productivity. There is no need to elaborate on thisphase of the subject. An annual consumption of approximately a million toneof lime and ground limestone, largely for use on "sour" soils, is sufficientevidence of the importance of soil acidity.

Soil Acidity as a Basis of Classification.

However, in considering the advisability of classifying soils accordingto their acidity, I think we should forget for the moment the agronomic im-portance of acidity. The purpose of a soil survey is to make a scientificclassification of soils and we should not be diverted from this purpose byin-mediate utilitarian considerations any more than a botanist would be di-verted in classifying plants or a mineralogist in classifying minerals. .Ifthis is borne in mind the advance in soil science should ultimately be greaterand the practical advantages accruing from soil surveys shot-id also be greater.A classification of soils will coordinate our knowledge of soils and assistin determining the best methods of soil treatment in proportion ae it islogical and scientific in a purely systematic cense. It seems. to me thenthe question of whether soils should be classified or. the basis oi acidityis merely a question of whether such a classification would bu as accurateand reasonable a classification, one as generally useful, as the system wenow have.

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1. In charge Chemical Investigations, Bureau of Soils, U.S.D.A.

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Soil acidity is one of the characteristics of a soil and as such mightbe considered in making a classification, but it is probably a minor char-acteristic from a systematic standpoint. Acidity in cultivated fields isa somewhat evanescent characteristic, the intensity of acidity varying withthe cultural treatment. A cultivated field may have one reaction today andanother a few years hence, depending on the liming and fertilizer treatmentit has received. The plots at the Rhode Island Experiment Station underdifferent methods of treatment have developed acidities varying fromP 4.5 to P 7.8.1 Doubtless 90 per cent of all the soils within the UnitedStates would fall within the range of acidity exhibited by this one field ofone soil type. Nevertheless, it is obvious to any one walking by these plotsthat the essential characteristics of the soil are unchanged so far as a soilclassification is concerned.

Of course, special classifications and groupings of soils nay be madeon any basis whatever, and such groupings may be more useful for specialpurposes than a classification which is truly a soil classification. It'would be a mistake, however, to abandon the true classification for sucha grouping or to confound one with the other.

Acidity tests in conjunction with Soil Surveys.

The question whether acidity tests should be made in the course ofsoil surveying is entirely distinct from the question whether these testsshould be made as a basis for soil classification. If soil acidity testsare not made while an area is being surveyed they should be made afterwards,because of the practical significance of these determinations. Also a system-atic collection of soil acidity data from definite soil types in connection .with a survey of the growth of certain crops on these types would probablyconsiderably increase our knowledge concerning the diagnostic value of suchtests.

The acidity method that should be used is important. There are somany different ones that it is impractical to dodge the question by suggest-ing the use of all of them. Most acidity methods are obviously empiricaland the uncertainty about their value is reflected in the fact that noneare now included in the methods of the Official Agricultural Chemists.

If a trial is to be made of acidity determinations in conjunction witha soil survey it seems to me that it would be best to employ the electro-metric or Colorimetric determinations of the hydrogen-ion concentration.This determination is readily marie; it is not empirical, but measures some-thing definite and the result obtained are therefore likely to prove ofvalue irrespective of how theories change regarding the nature or mechanismof the correspondence between crop growth and soil acidity.

While a determination of the PH or hydrogen-ion concentration, showsthe degree of acidity actually existing in a water extract of the soil, itis only a measurement of -Lhs intensity factor of soil acidity. A determina-*™ "™~"™*"™*^«»""^««^*««*""«»«»«fcll«M*«™—— 1«WMM ••••,« MM ~-M*W«««*M«>M*--«w__^M«V«**WM^«»WWWH**V »•• M *• ^ •»• •

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tion of the quantity factor of soil acidity should also be made. The quantityfactor of soil acidity cannot be defined precisely until we know just how thesoil solution becomes possessed of an excess of hydrogen ions. It might beexpressed, however, in terms of the quantity of lime or other base that mustbe added to the soil to bring it into such a condition that it would imparta neutral reaction to a water extract. The quantity factor of soil acidity

in tb-'S sense is probably equivalent to some of the old "lime requirement"determinations, such as the Veitch determination.

The intensity and quantity factors in soil acidity are of course dis-tinct and one would not expect to find a parallelism between them. Asa matter of fact, comparisons that have been made by Johnson^ and Wherry^show that the PH of the soil solution does not vary with the lime require-ment of the soil. It is to be expected that crop growth will show a greatercorrelation with the PH than with the lime requirement. In other words, thePH will prove to be the indicator of the need of liming. Ths limitation ofpotato, scab to soils less acid than PH 5.2, established by Gillespie andHurst,d and the correspondence between natural flora and PH of the soil,shown by Wherry, * are indicative of this and at the same time are suggestiveof what may be found for staple crops.

However, in spite of all the investigations that have been made ofliming it is yet to be determined whether there is a close correlationbetween the PH of different soils and their productivity for the importantcrops. Any systematic collection of data to determine whether there aredefinite intensities of acidity which are optimum for the growth of wheat,corn, cotton and alfalfa, etc. in normal soils should certainly be encour-aged.

If the reulsts of such an investigation were positive, u lime require-ment of the soil could doubtless be worked out which would tell how muchlime to use for each crop on a given soil. This lime requirement determina-tion would not be one quantity, as in the old determinations, but a seriesof quantities—a curve showing the line required to bring the soil toa series of P values.

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1. Johnson, H. W. Soil Science, Vol. 13, pp. 7-22.2. 'Wherry, E. T. Jour. Washington Acad, SCI, Vol. 13, pp. 97-102.3. Gillespie, L. J., and Hurst, L. A., Soil Science, Vol. 6, pp. 219-236.4. Wherry,"E. T., Smithsonian Report for 1920, pp. 247-268.

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