Tests of Plants and Soils

by MICHAEL PEECH and HANS PLATENIUS

CHEMICAL tests of soils and plants give farmers valuable tools for measuring the most profitable returns from fertilizers and diagnosing causes of crop failures.

Ever since the German professor, Justus Liebig, published his epochal book, Organic Chemistry in its Application to Agriculture and Physiology, in 1840, soil and plant scientists have been working to perfect these chemical analyses. They have made much progress toward their goal of better techniques for making the tests and interpreting the results, but they have not yet reached it. Soils and plants are wonderfully complex and represent the beginnings and results of intricate forces of nature. Testing one or the other, therefore, is not a simple matter of taking a test tube or litmus paper to a field and in a minute learning all of nature's complicated secrets.

Rapid chemical soil tests or "quick soil tests," as they are commonly called are simple adaptations of chemical methods of analysis, designed to measure the amounts of readily soluble or "available" plant food elements in the soil. Obviously there must be a specific chemical test for each of the plant nutrients or constituents that are being examined a test for soil acidity, nitrogen, potassium, phosphorus, calcium, magnesium, manganese, and so on.

Detailed descriptions of the different types of extracting solutions and the techniques for making the chemical soil tests for the different nutrient elements, in current use, may be found in the literature listed in the bibliography. The usual procedure is to shake a weighed sample of soil with a measured volume of the extracting solution.

One of the extracting solutions commonly employed for the purpose is a mixture of dilute acetic acid and sodium acetate strongly buffered at pH 4.8. After shaking for a definite interval of time, this soil suspension is filtered and the tests for the different nutrient elements are then made directly on the clear soil extract thus obtained. The amount of a given nutrient element in the extract is determined by treating a portion of the extract with the proper reagent and comparing the color or cloudiness produced with that of a series of standard solutions carried simultaneously through the same procedure. In the more simplified tests, standard color and turbidity charts are used for comparison.

Unlike the classical, more accurate methods of soil analysis that involve a number of different extractions and laborious analytical separations, the rapid chemical soil tests are made directly on separate portions of a single soil extract by means of colorimetric and turbidimetric methods without prior treatments or analytical separations, although few of the inorganic and organic reagents employed in these tests are specific enough to permit their direct use in the presence of other interfering elements in the extract. The chief advantage of these chemical soil tests over the classical and the more conventional laboratory methods lies only in their simplicity and the rapidity with which the individual tests can be carried out. Indeed, it is this feature that makes them especially well suited for practical routine soil testing; for that purpose the cost of using the more tedious classical chemical methods would be prohibitive.

Some of the tests in current use, however, have been oversimplified to the extent that they are no longer sufficiently reliable or accurate and, in fact, may be quite misleading. It is often implied that there are so many biological and environmental factors involved in the practical use of the soil tests that relatively little reliance can be placed on the results of the tests, however accurate they may be, and hence, no great accuracy is required. Obviously, such a wrong viewpoint may be carried to the point where the results become so questionable as no longer to justify the expenditure of time in making the tests.

The poor correlations often observed between the results of chemical soil tests and the crop responses to fertilizers often may well be attributed to the analytical errors inherent in the soil tests. Such errors may be due to the interferences by other elements or constituents present in the extract, color fading, the slow deterioration of some of the reagents, as well as to the significant influence of the variation of temperature and time upon the final color or turbidity developed in some of the tests.

The directions for making these tests must be followed precisely. A slight deviation from the proper procedure introduces serious errors. Thus, even simplified soil testing presupposes some knowledge of the delicate and sensitive chemical reactions involved in the tests. Many of these tests can be properly made only by trained technicians in the laboratory.

Moreover, the results of the tests can be interpreted only by a trained agricultural worker, who is familiar with local farm practices, chemical and physical characteristics of the soils, plant-nutrient requirements of different crops, and the intricate plant-soil relationships. The results of the chemical soil tests must be thoroughly calibrated against crop responses to fertilizers on different soil types and under different climatic conditions. The influence of the climatic factors in determining crop responses to fertilizers even on a given soil type has been frequently overlooked. Other limiting factors besides lack of plant nutrients, such as poor drainage and lack of moisture as well as the value of the crop, must also be considered in determining the kind and the amount of fertilizer that may be profitably applied.

In contrast to the more conventional ultimate method of chemical analysis, which seeks the total amount of a given element present in the material analyzed, the chemical soil test should measure only the portion of the total supply of the nutrient element that the plant can utilize during its relatively short growing period. Or, at least, the fraction of the nutrient element that can be extracted by the chemical soil test should correlate with known crop responses to the application of that element.