NEW CONCEPTS IN THE CHEMISTRY OF SOIL FERTILITY1

R. H. Bray2

The problem of determining the ferti-lizer needs of a soil by means of chemicalmethods has received considerable attentionwithin recent years. This has been due tothe introduction of new methods designed forthis purpose. Some of these methods arebased on new concepts as to how this shouldbe done which are at variance with the olderconcepts in this field. It is the purpose ofthis paper to explain these new concepts, tocompare them with the older concepts and toshow how the methods based on these new con-cepts have been made practical and how theycan be extended to apply to numerous new con-ditions.

Historical

The study of soil fertility from thestandpoint of fertilizer needs has followed,mainly, three general methods: the experi-ment field methods, the pot culture method,and the chemical study of the soil and theplants growing in the soil. Each of thesemethods is more or less dependent on the re-sults of the other methods for the interpre-tation of the results secured. A well-bal-anced soil fertility study, therefore, re-quires that chemical studies of the soil bemade along with the experiment field or potculture studies.

The first chemical studies concernedthemselves with the total analysis of thesoil but no practical correlations betweenthe total amount of an element and the needof the soil for- that element were secured.Toward the end of the last century the con-cept of measuring the availability of thefertility elements in the soil became fullydeveloped. The citric acid solubility methodof Dyer is the most striking example of thisschool of thought. This school of thoughtdeveloped mainly as a result of the lack ofsecuring any practical correlations betweenthe total amounts of an element present andthe crops' need for that element in the soilconcerned. The results secured were much

more promising. Rather broad correlationsbetween the amounts of the elements extractedby such methods and the crop response to add-ed elements were secured. These studiesfailed to lead to specific correlations andno recommendations for the general use of anyof these methods as a practical indicator ofsoil fertility needs resulted. However, thisschool of thought greatly stimulated interestin soil chemistry, especially in that branchof chemistry attempting to measure the fer-tility needs of soils. The concepts on whichthis school of thought was based involved, notonly the idea of measuring the availability ofthe fertility elements in the soil but alsothe procedure to be followed in so doing. Itwas considered that the chemical method of ex-traction should simulate the feeding of theplant roots and that the amount of the elementextracted by such a solution should be a meas-ure of the availability of the element. Inthe light of the newer knowledge of soil chem-istry, these concepts and the methods whichresulted from them appear inadequate, although.they were based on apparently sound reasoningin the light of chemical knowledge at thattime. It is now> known, for example, thatbase-exchange equilibria in the soil preventsthe accurate measurement of the available Ca,Mg, and K, occurring as replaceable bases bymethods which try to simulate plant feedingpower.

The New Concepts

The new school of thought which hasdeveloped recently has the same objectives asthe old school—that is, the establishing ofmethods for determining the fertilizer needsof soils.

The concepts, upon which the new schoolbases its method of study can be stated in afew sentences. Usually more than one form ofan element occurs in the soil at any one time.Each form contributes to a greater or lesserextent to feeding by plant roots. The signif-icance of each form to the growth of each crop

•"•Contribution from the Division of Soil Fertility, Department of Agronomy, Illinois Agricultural ExperimentStation, Urbana, Illinois. Published with the approval of the director of the station.

Assistant Chief in Soil Survey Analysis.175

176 SOIL SCIENCE SOCIETY PROCEEDINGS 1937

under varying conditions can be establishedby studies of plant growth and soil chemis-try. The form or forms found to be of mostsignificance to the immediate growth of theplant can be measured chemically and thisinformation can serve as a partial basis fortreatment recommendations. This is a briefstatement of the concepts of the new schoolof thought in the chemical study of soil fer-tility.

This is a departure from the oldschool of thought in many particulars. Noattempt is made to measure the composite"availability" of all the forms of a givenelement present nor is the feeding power ofthe plant roots simulated in the extractingsolution used. Instead, the extracting so-lutions to be used are those- which can ex-tract all of that form or forms of the ele-ment being tested for and which have, there-fore, a purely chemical basis. The form orforms to be tested for must be determined bychemical and fertility studies using thetypes of crops and soils on which the testsare to be applied. Without any knowledge asto the availability of each form to eachcrop, an interpretation of the test would beimpossible, even though the proper form wasbeing measured correctly. The new school,therefore, requires that to be of most prac-tical use, the methods or tests must not onlymeasure the significant forms of the elements,but also correlations must be established be-tween the amounts of these forms of the ele-ments present and crop response to the fer-tility elements applied.

A practical test or method is, there-fore, one which has been calibrated againstactual crop response under the conditionsconcerned. Attest practical for one condi-tion will not necessarily be practical foranother condition until the necessary corre-lations have been established.

Designing a Test to Meet the New SchoolRequirements

Designing a test or method meetingthe requirements of the new school is not aneasy matter. Many years of experimentalfield work must be carried on accompanied bychemical soil studies. It is the soil chem-1st who has experiment field soils with knownhistories of response and non-response onvarious soil types who has the facilities todevise such methods. The first step is tocompare responding soils with non-respondingsoils in chemical studies, the form or formsof most significance in the response are de-

termined. This is the first and most impor-tant step. The studies must be broad enoughto eliminate chance agreements, and all pre-conceived ideas as to what forms should bemeasured must be eliminated from the study.For example, a good correlation between the Kin the soil .solution and K needs and response,might easily be obtained under very limited .conditions, and yet the measuring of the K inthe soil solution as a general means of esti-mating K needs has been shown to be impossi-ble.

The second step is to correlate theamounts of these forms of a given elementpresent in the soil with the crop response tothe elements applied as .fertilizers. Theforms of the elements most available to cropfeeding or most significant in crop feedingmay be more or less similar in all soils. Theamounts of these forms which must be presentwhen no further response is obtained, however,will vary with the crop and the productivelevel of the soil type as limited by all theother conditions of growth. Information con-cerning individual crop requirements for, andresponse to, these forms and the soil's gen-eral ability to produce the crops is neededfor such correlations, and can only be ob-tained by some form of experiment field study.

The third step is to devise a test orlaboratory method for the measurement of thesignificant forms of the given element as dis-cussed above. Such a method should measurethese forms quantitatively—at least, roughlyso—and one should be sure that it is not in-terfered with by other substances in the soilor in the reagents used. For example, calciumcarbonate in a soil interferes with the appli-cation of the easily acid-soluble phosphorustest because it neutralizes the extractingacid and because in dissolving it may liberatephosphorus not as available to the plant asthe easily acid-soluble forms.

In order to be of practical value to afarmer, the results of a test must be applica-ble under the soil management and croppingsystems used. This requires that the signifi-cant forms of a given element be measured, asto amount, around the transition range betweencrop response and non-response to fertilizerscontaining the element in question. Suppose,for example, that a method for replaceable po-tassium could distinguish between differentamounts accurately up to 60 pounds an acre,above which all readings were indistinguisha-ble "highs." The results would be misleadingto a corn belt farmer, for whose crops anyamounts less than 90 to 100 pounds an acre aredeficient, and amounts up to 140 pounds anacre are in the transition range between

NEW CONCEPTS IN THE CHEMISTRY OF BOIL FERTILITY 177

deficiency and sufficiency.It is obvious that in this example

the method must distinguish sharply betweendifferent amounts of potassium from below 90to above 140 pounds. Even though all testsbelow this range read uniformly low and alltests above the range read "high" the teststill tells the farmer what he needs to know.On the other hand, suppose a method distin-guished different amounts accurately betweengOO and 600 pounds an acre of replaceable po-tassium and that below or above this rangethe readings were uniformly "low" or "high"respectively. Such a test would be uselessfor corn belt crops because the reading rangeis entirely above the significant range forthese crops. But it might be exactly rightfor some other crops, such as potatoes, pro-vided the transition from response to non-response of that crop to potash fertilizersoccurs at soil levels between 200 and 600pounds of replaceable potassium.

Adjustments of tests for specialcrops or conditions so that the reading rangeof the test falls within the range which issignificant in response and non-response,must be made in such a way that the quantita-tiveness of the test is not altered. For ex-ample, in the case of tests for replaceablepotassium, the higher reading range can beobtained by using less soil for a given vol-ume of extracting solution or by diluting theextract obtained and multiplying the resultby the dilution factor. What might appear tobe similar results could be obtained for verylimited conditions by using a weaker salt oracid solution for the extraction, a procedurewhose error has already been discussed in aprevious paper (4).

When all the preceding requirementshave been met, the soil test or laboratorymethod is ready to be used in practical ap-plications.

Some Practical Applications of theNewer Concepts

To meet the requirements of the newschool of thought in measuring fertilizerneeds by chemical examination of the soil isnot an easy matter or one for only casualstudy. Practical field tests meeting all therequirements discussed above have been devel-oped, however, and put into practical use.Unfortunately, the application of these testsin practice has not always been made by ad-herents to the new school of thought. Theapplication of the old school ideas of meas-uring availability to the use of the new

school tests has resulted in modification ofthese tests in such a way as to make them en-tirely useless in some cases.

One of the first tests for the ele-ments of major importance is a test for ni-trates described by Morgan, in 1930 (6). Thisis the nitrate test used in various soil-testing kits. It gives an indication of thelarger variations in amount, although it isnot very sensitive in separating soils varyingby smaller differences. The value in usingany nitrate test depends on one's ability tocorrelate the amount found with crop needs,even though the correct form is being measuredaccurately. Numerous factors must be consid-ered in interpreting the test, such as theamount of recent and expected future rainfall,and the crop and the stage of its growth. Nogeneral correlation or interpretation of thenitrate tests has been worked out, althoughthey are of real value in showing the presenceof sufficient or excessive amounts, low testsbeing much harder to interpret.

The first calibrated field test amongthe'major elements recommended for generalpractical use, was a field test for easilyacid-soluble phosphorus published by thewriter in 19S9 (l). This test meets all therequirements of the new school for the condi-tions for which it was recommended—that is,it is a measure of the form or forms of phos-phorus responsible for the variations in re-sponse for the soils and crops concerned andthe system of management and treatment used,and permits direct, recommendations as tophosphate needs for these conditions. Its usehas been invaluable in the corn belt in show-ing where phosphorus is most diflcient andhundreds of thousands of acres have alreadybeen tested and mapped for phosphorus defi-ciencies by the test. The general problem ofmeasuring "available" phosphorus is discussedin detail in a paper to be presented later atthese meetings (5).

When first published, some objectionsto the test were made, based mainly on the oldschool line of reasoning. The test was ap-plied to truck crop soils or greenhouse soils,for example, and no exact correlations werefound. It was natural to take it for grantedthat, since the test measured "available" phos-phorus for the grain and hay crops of the cornbelt, it could be used for any kind of soil, cropor crop management system. A "high" reading wastaken to mean a high amount of available phos-phorus for any crop on any soil for any manage-ment system. This, of course, -was the naturalinterpretation under the old school conceptsof measuring availability and the statementmade by the writer that the calibration was

178 SOIL SCIENCE SOCIETY PROCEEDINGS 1937

for the soils, crops, and management systemsconcerned was often overlooked (l). Underthe new school concepts, the first study ofa suggested test to be applied to new condi-tions not included in the original calibra-tion should start at the bottom and determinewhether or not the test is measuring the formor forms of most importance for the new con-ditions concerned, with no interference fromthe chemical properties of the soil. Thereason for the lack of good correlationsshown by the easily acid—soluble phosphorustests with certain crops is mainly due to thefact that appreciable amounts of highly solu-ble phosphorus must be present for bestyields of these crops so that both easilyacid-soluble and water-soluble forms must beconsidered. • The difficulty involved here isthat no test for the highly soluble forms hasbeen developed, except for greenhouse soilswhere fixing capacity for P has been overcomeby liberal soluble phosphate applications.

The next step should be to see wheth-er the test is giving, a quantitative measureof the significant form or forms. Actually,the phosphorus test recommended by the writerdoes not extract all the easily acid-solublephosphorus, but it does extract a proportion-ate part so as to give an approximately quan-titative measure of the total present. Thevalues obtained with the test have been cor-related with Truog's laboratory method foreasily acid-soluble phosphorus. This corre-lation shows that a "high" test is 75 pounds;a "medium" test, 50 pounds; a "doubtful" or"slight" test, 35 pounds, and "low" was 25pounds or less per acre—2,000,000 pounds ofsoil.

The third and final step would be tosee whether or not a correlation can be es-tablished between crop response and test val-ue. If a correlation cannot be established,it is probably due to too few cases being in-cluded in the study or to the fact that thefield study is influenced by other variablesthan the element studied. For example, onecannot use a test to guarantee a response,since in so doing, the weather conditions,etc., would also be guaranteed. The condi-.tions used in the calibration must be broadenough to smooth out such variables. Actual-ly, if the first two requirements have beenmet, the third should be possible, and anylack of correlation is due to the lack ofadequate crop re.sponse studies to establishthe calibration.

The principal danger in a wholesalerecommendation of even well-calibrated soiltests is that they will be applied to numer-ous conditions not included in the original

calibration. Too often the original calibra-tion is accepted as having too broad applica-tions. It is necessary for those who recom-mend a test to the farmer to understand thecalibration and its limitations. Actually,of course, the tests are used in a practicalway for somewhat broader conditions thanthose involved in the correlation, such usebeing dependent on a specialized knowledge ofthe soils and crops concerned.

Numerous similar tests for phosphorushave since been.described, based on acid ex-traction. In some of these the acid solutionis too weak for satisfactory use under corn-belt soil conditions. Weakening the acid so-lution lessens the proportionate amount ofeasily acid-soluble phosphorus extracted sothat, in general, soils very high in thisform are the only ones to give a "positive"test and the significant levels of easilyacid-soluble phosphorus are not being includ-ed in the test's measuring range. While the.further study of the weaker acid solutionsshould not be discouraged, they cannot be ac-cepted for general use without proper corre-lations.

In 1931 the writer introduced a fieldtest for total replaceable potassium and sug-gested its possible use as a field test for"available" potassium (2). This test, aspublished, did not meet all the requirementsfor a calibrated field test, although it didmeasure the total replaceable potassium. Ithad not been shown that the replaceable K wasthe most significant form or that it could becorrelated with crop response. At about thetime this paper appeared in print, however,these requirements had been met and the testwas recommended for general practical use forthe conditions involved in the calibration.The general subject of replaceable K and Kavailability has been discussed in a formerpaper (3).

The correlation between replaceaole Kand crop response for corn-belt conditions isremarkably exact. The "low" test results aregood evidence of a potash deficiency and thetests have been used to predict future re-sponses on soils which have not previouslyresponded to potash fertilization. About 140pounds of replaceable K per acre in 2,000,000pounds of soil, has been found to be suffi-cient for average crop yields on otherwisewell—treated land.

Recently the writer published a methodin which a sodium perchlorate solution (25 percent) was substituted for the acid sodium ace-tate solution formerly recommended and showedthat practically quantitative accuracy couldbe obtained with the method which now gives

NEW CONCEPTS IN THE CHEMISTRY OF SOIL FERTILITY 179

readings in pounds of K per acre for shortintervals from 40 to 300 pounds per acre.Readings above SOO are possible by making thenecessary dilution of the filtrate (4). Atthe same time it was shown that certain othertypes of extracting solutions which had beenrecommended in some cases for soil tests,could neither extract nor give a measure ofthe replaceable K presenjt. Although in gen-eral, such tests appear to follow the origi-nal method, the extracting solution has beenso changed as to make the tests unreliable.

Tests for the Other Plant Food Elementsor Poisons

No calibrated tests for the otherelements have been produced. Numerous quali-tative or practically quantitative tests havebeen described which are useful in certainways, but they have not met all the require-ments of the new school for calibrated tests.They are, however, valuable as suggestionsfor further study and sooner or later suchcalibrations will probably be made, providedthe right forms of the respective elementsare being measured.

Perhaps the most valuable of these isthe test for replaceable magnesium, sincemagnesium has been found to be deficient oncertain soils in the east. Whether the pres-ent tests are actually calibrated or not is aquestion, but certainly only a rough calibra-tion may be needed, because, where relativelysmall amounts are found, a recommendation forthe use of magnesium is logical, in view ofthe small cost involved.

Most of the tests indicate relativelylarge or small amounts of the forms measuredand with many of them a positive test may in-dicate that no deficiency exists. Unfortu-nately, a negative test does not necessarilyindicate a deficiency because the necessarycalibration is lacking. As far as the tests

indicating harmful amounts are concerned,again definite correlations are lacking, butthe tests may be useful in eliminating ques-tionable conditions. Amounts normally pres-ent on highly productive soils might be con-sidered as not harmful, while larger amountsmight be viewed with suspicion. Most of thesuggestions for the use of these tests arebased on some such generalities, which, how-ever, are probably as good as the presentstate of knowledge permits. Nevertheless,the establishing of a real calibration is tobe desired, and once a calibration is estab-lished for one condition, it makes the appli-cation to other conditions much easier.