G Yhemical Tests of Soils and Plants Give Farmers Valuable

Tests of Plants and Soils by MICHAEL PEECH and HANS PLATENIUS

GyHEMICAL 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 Physi- ology, 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 com- plex 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 diflferent types of extracting solutions and the techniques for making the chemical soil tests for the difïerent 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 bufiFered 583 584 YEARBOOK OF AGRICULTURE 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 in- volve 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 foUow^ed precisely. A slight deviation from the proper procedure introduces serious errors. Thus, even simplified soil testing presupposes some knowledge of the deli- cate 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 TESTS OF PLANTS AND SOILS 585 and physical characteristics of the soils, plant-nutrient requirements of diflfercnt 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. It is well known that the total supply of a given plant nutrient in the soil, as determined by breaking down the soil completely in the course of a total chemical analysis, is no measure of the amount of the plant nutrient element that is at the disposal of the plant. The more soluble portion of the total supply of a given element that the plant can utilize for best growth is commonly called the available supply or the available form, even though it is realized full well that such a division of the total supply of any nutrient element in the soil into two categories, the available and the unavailable form, is quite arbitrary and often very diíTicult. Yet, before the results of the test can be of any value in predicting the fertilizer needs of the soil, the extracting solution employed in the chemical test must distinguish accurately those forms of plant-nutrient elements that are available from those that the plant roots take up only with difficulty. Indeed, the success of the chemical soil tests depends largely on how well the tests can simulate the ability of the plant roots to obtain from the soil sufficient amounts of the dififerent nutrient elements to meet requirements for normal plant growth. Despite the tremendous amount of work done and the progress already made, it is not possible at present to determine by chemical tests the exact grade or the most profitable amount of fertilizer that may be applied to a given soil for a specific crop even though such tests have been successfully employed as a guide to fertilizer recommendations. But it will be useful to examine the limitations of the tests when we attempt to use them to predict the need for nitrogen, phosphorus, and potash—the three principal constituents of fertihzers. The greater part of the first nutrient element, nitrogen, is intimately associated with the organic matter in the soil. While it is in that organic 586 YEARBOOK OF AGRICULTURE

form the plant cannot use it. As it gradually decays, this organic form of nitrogen is converted into the water-soluble or available forms ( am- monia and nitrate) that can be readily absorbed by the plant roots. Just how rapidly the nitrogen in the soil organic matter can be changed over into these simple water-soluble forms depends on the nature of the soil organic matter and the microbiological activity, as determined by soil moisture, soil temperature, and other soil factors. Because the nitrogen-supplying power of the soil is closely associated with the break-down of the organic matter, the determination of the total amount of nitrogen in the soil can be of Htde value in predicting w^hether a nitrogen fertilizer can be apphed profitably unless wc know the rate at which the soil organic matter is decomposing. Certainly the native soil organic matter, which has resisted decomposition for years, will not release its nitrogen as rapidly as the freshly incorporated organic matter, say after plowing down of a legume sod or a green-manure crop. The obvious alternative of determining the water-soluble forms of nitrogen, which is actually the procedure followed in rapid chemical soil tests, also has its limitations. The water-soluble form of nitrogen is subject to leaching by rains; the nitrate-nitrogen content of the soil, therefore, fluctuates considerably during the growing season. The absence of nitrate nitrogen in the soil as revealed by the chemical soil test, following a heavy rain, docs not necessarily indicate acute nitrogen shortage if the rate at which the soil organic matter is breaking down into the simple water-soluble forms of nitrogen is sufficiently rapid to replenish the losses due to leaching and crop removal. Thus, a low supply of available (nitrate) nitrogen at any one time, as may be shown by the chemical soil test, is no assurance that a nitrogen fertilizer can be applied with profit. The ability of certain crops like legumes to utilize atmospheric nitrogen must also be considered in predicting whether the use of nitrogen fertilizer may be profitable. The chemical soil test for potassium, must also distinguish accurately between the available and the unavailable forms of potassium. The w^ater-soluble as wxU as the exchangeable potassium—that is, the part of the soil potassium that is extractable by dilute acids and neutral salt solutions—is considered as the available form. Most of the mineral soils in the eastern half of the United States that contain as much as 20 tons of potash' an acre in the plowed layer have been found to give a profitable response to an application of only 200 pounds of muriate of potash per acre. The reason for this, of course, is that the potassium in such soils occurs mosüy in mineral forms that are too insoluble to supply potassium at an adequate rate for normal crop requirement. Fortunately, the less soluble mineral forms of potassium gradually con- vert into the available form (water-soluble and exchangeable forms). The presence of an adequate supply of available potassium in the TESTS OF PLANTS AND SOILS 587 soil at any one time as determined by the chemical soil test is a good indication that potash fertilization is not likely to give a profitable return. On the other hand, different soils with a low available potassium content do not give equally good responses to applications of potash. Al- though the available potassium supply in the soil at any one time, as determined by the chemical test, may be relatively low, many investigators have found that the amount of available potassium in certain soils remains remarkably constant, even upon intensive cropping. This would indicate that as the plant removes the available potassium from the soil, some of the difficultly soluble forms of potassium are converted into the available form. Thus when the supply of available potassium in the soil is replenished from the difficultly soluble forms rapidly enough to keep pace with the removal of potassium by the crop, the potassium- supplying power of the soil may be considered adequate even though the amount of available potassium at any one time, as revealed by the chemical test, may be low. Similar conditions apply to the test for determining available phosphorus. Many mineral soils having a total phosphorus content of more than 1,000 pounds to the acre in the surface layer respond well to as little as 50 pounds of phosphorus an acre applied in the form of superphos- phate, especially if no phosphatic fertilizer has been previously used. This apparent discrepancy is due to the fact that most of the native soil phosphorus is tied up in the form that is not available to plant roots. Even the applied phosphorus is tied up so strongly by some soils that, unless a sufficient amount is added to satisfy at least partly the phosphorus-fixing capacity of the soil, little or no crop response can be observed even though the soil is deficient in phosphorus. Soil chemists have been searching for a chemical phosphorus test that would be sufficiently specific to differentiate the available forms of phosphorus from the difficultly available forms and that would thus predict accurately responses of different crops to phosphorus fertilization on different soils. Such an ideal phosphorus test is yet to be found. Some of the chemical soil tests for phosphorus in current use, however, give fairly reliable results when properly correlated with crop responses on different soil types. The tests for some of the constituents may be considered quite satisfactory. The soil acidity test, for example, which can be readily made even in the field, is now extensively employed as a basis for lime recommendations. But, here again, the translation of the results of the soil acidity test (pH of the soil) into the amount of lime that needs to be applied to grow a certain crop requires some knowledge of soil characteristics and plant requirements. Despite these limitations, the chemical soil tests have been shown 588 YEARBOOK OF AGRICULTURE

to be of great value as a guide to fertilizer recommendations, especially in detecting extreme deficiencies and toxicities of plant nutrient elements in the soil. The chemical soil tests, for example, can show that continuous, heavy fertilization over a period of years may result in accumulation of one or another plant nutrient element in sufficient amount to warrant the reduction or even omission of that constituent in the fertilizer without sacrifice of yield or quality of the crop. The soil problems in commercial greenhouses are often associated with accumulation of excessive and injurious amounts of fertilizer salts. Such a condition can be readily detected by chemical soil tests. Chemical soil tests are also useful as an aid to diagnosing crop failures. Admittedly, crop failures may be due to many difiierent factors other than lack of adequate supply of plant nutrients. Poor drainage, lack of moisture, and insect or plant-disease damage may be the cause of the trouble. Diagnosis from the results of chemical soil tests is often diflicult, but some clues as to possible causes of the trouble may be obtained largely by the process of elimination. These tests are now used extensively by many State agricultural experiment stations, principally in the central-western area, Corn Belt sections, and eastern Atlantic region, as an aid in furnishing advice to farmers and growers regarding fertilizer use and soil management. Many States render this service free of charge to resident farmers, growers, and public agencies of the State. Special soil containers with detailed instruc- tions for collecting the soil sample printed on the container are provided for this purpose by some State soil-testing laboratories. It is imperative, of course, that the soil sample be properly taken in the field. When submitting the soil samples for testing, the farmer is also requested to fill out special blank forms, giving pertinent information regarding the field and the soil problem. This is necessary for proper interpretation of the results of the tests and for making the final recommendations.

Plant-Tissue Tests

Because of the limitations of soil tests as a guide to fertilizer recommendations, some investigators prefer to use the chemical composition of the plant itself. This procedure makes allowance for the variations in different crops in their nutritional requirements and in their ability to utilize the nutrients in the soil. Plant composition also reflects changes in the nutritional status of a crop as a result of varying climatic conditions. For instance, deficiencies of nitrogen and magnesium occur more frequently in seasons of high rainfall than in dry years. Boron and manganese deficiencies, on the other hand, occur more often in dry seasons than in wet seasons. Obviously, these complicating factors should be TESTS OF PLANTS AND SOILS 589 eliminated if the diagnosis of nutritional deficiencies is based on the composition of the plant itself as it grows under natural conditions in the field. To ascertain nutrient-element deficiencies at an early stage of growth, chemical analyses of the entire plant or of certain parts of the plant have been used. The principle involved is the same as that of the well-known Neubauer test except that the tests are made for each specific crop under field conditions. This procedure is based on the following assumptions: Nutrient deficiencies in the soil must be reflected accurately by the correspondingly low concentrations in the plant at an early stage of growth ; differences in composition of deficient and normal plants must be large enough to be easily measured; critical concentrations—the minimum concentration of the diflfcrcnt elements in the plant tissue below which growth is retarded—must be reasonably constant under diflferent soil and climatic conditions to serve as a basis for comparison. Unfortunately, little work has been done to verify the correctness of the assumptions. Many investigators have demonstrated that deficiencies of nitrogen and potassium are reflected in correspondingly low concentrations of the elements in the plant. But it has not been shown conclusively that analysis of tissues reveals these deficiencies at an earlier stage of growth than can be recognized by visual symptoms of the foliage. Also, a deficiency of phosphorus in the soil may eflPect a definite reduction in yield of the crop without causing an appreciable lowering of the phosphorus content of the plant tissue. The determination of the fertilizer needs by chemical plant-tissue tests has been most successful with fruit trees and crops that require more than a single season to mature, such as sugarcane and pineapples. It is more difficult, if not impossible, to establish definite critical concentrations for short-season crops, especially where such crops are grown under widely different climatic conditions. The chemical methods of plant analysis vary from simple plant-tissue tests carried out in the field to careful laboratory analysis of the ashed plant material. Field tests are little better than qualitative tests and they are useful only for ascertaining extreme deficiency or abundancy. The tests for nitrates by means of diphenylamine is perhaps the most useful of the field tests. Failure to develop a deep blue color when this reagent is added to the split lower petiole of many crop plants is a good indication of nitrogen deficiency. On the other hand, a positive test is no proof that the plant contains sufficient nitrogen for maximum growth when applied to such crops as potatoes or tomatoes, which normally have a high concentration of nitrate in the tissue. Tissue tests arc now made on a purely experimental basis. Again, as in soil tests, there is a complete lack of standardization with respect 590 YEARBOOK OF AGRICULTURE

. to sampling procedure, extraction methods, and analytical methods used. A great deal more fundamental work is needed to furnish the answer to questions such as these: What part of the plant should be sampled for analysis; does the soluble fraction of the nutrient element constitute a better index of the nutritional status of the plant than the total quantity of the element present; are differences in composition as a result of nutrient deficiencies large enough to overcome sampling errors; and, finally, is it possible to state the absolute, critical concentration for each nutrient and for each crop under different climatic conditions? Not until we have answers to these questions can we finally appraise plant-tissue testing. Deficiency Symptoms

The simplest and in many respects the most satisfactory method for ascertaining the need for certain nutrients is to recognize deficiency symptoms by the color of the foliage, the size of the plants, and their growth habit. A light-green or yellowish color of corn plants indicates to every farmer the lack of nitrogen in the plant. Symptoms of nitrogen deficiency may be temporary on cold, wet soils and may disappear in early summer under more favorable soil conditions. Color charts are now being used by growers to measure the color of the leaves of apple trees. It serves as a fairly dependable method for ascertaining the nitrogen requirements of apple trees. No doubt this procedure could be used to advantage with many other crops. Symptoms of deficiencies o.f other nutrients, like potash, magnesium, manganese, and boron, are now well established. Phosphorus deficiency remains one of the most diflicult to recognize. Excellent descriptions of commonly occurring deficiency symptoms in the important crops have been published in Hunger Signs in Crops, It should be pointed out, however, that an accurate diagnosis by means of deficiency symptoms requires considerable experience and close observation. Faulty diagnosis is not at all uncommon. For instance, the chlorotic leaf margins of alfalfa caused by leaf hopper injury are easily mistaken for boron deficiency. It also is difficult to distinguish between the symptoms of magnesium and manganese deficiency. Nevertheless, when used by an experienced observer, diagnosis by means of deficiency symptoms is one of the most useful methods of determining the fertilizer needs of crops. Thus soil tests, plant-tissue tests, and deficiency symptoms are useful for identifying nutritional disorders in crops. Each technique has its limitations, it is true; but when properly used, especially in conjunction with each other, they can be of help as guides to fertilizer recommendations and for finding out why crops fail. TESTS OF PLANTS AND SOILS 591

THE AUTHORS

Michael Pcech is professor of soil science at Cornell University, Ithaca, N. Y. A native of Canada, he was graduated from the University of Saskatchewan in 1930 and completed his graduate studies in soil chemistry at Ohio State University. Since 1933 he has been doing research in and teaching soil chemistry at several colleges and State agricultural experiment stations. Dr. Peech has made extensive investi- gations of the chemical properties of soils in relation to their inherent fertility and response to fertilizers, particularly in an effort to develop rapid chemical soil tests for determining the fertilizer needs of soils. Hans Platenius is associate professor of vegetable crops at Cornell University, Ithaca, N. Y. He had his early training in agricultural chemistry in Germany and later continued his graduate work in plant sciences and biochemistry at the University of Nebraska and at Cornell. As a member of the research staff in the department of vegetable crops at Cornell University, Dr, Platenius has spent much time in recent years making a systematic study of the value of chemical analysis of plants for diagnosing deficiencies of nutrient elements in vegetable crops.

FOR FURTHER READING Bray, Roger H. : Soil-Plant Relations: I. The Quantitative Relations of Exchangeable Potassium to Crop Yields and to Crop Response to Potash Additions, Soil Science, volume 58, pages 305-324, 1944. Bray, Roger H., and Kurtz, L. T.: Determination of Total, Organic, and Available Forms of Phosphorus in Soils, Soil Science, volume 59, pages 39-45, 1945. Carolus, R. L. : The Use of Rapid Chemical Plant Nutrient Tests in Fertilizer Deficiency Diagnoses and Vegetable Crops Research, Virginia Truck Experiment Station, Bulletin 98, pages 1530-1556, 1938. Chandler, Robert F., Pcech, Michael, and Bradfield, Richard: A Study of Techniques for Predicting Potassium and Boron Requirements of Alfalfa. I. The Influence of Muriate of Potash and Borax on Yield, Deficiency Symptoms, and Potassium Content of the Plant and Soil, Soil Science Society of America, Proceedings, volume 10, pages 141-146, 1945. Colwell, W. E., and Lincoln, Charles: A Comparison of Boron Deficiency Symptoms and Potato Leafhopper Injury on Alfalfa, American Society of Agronomy, Journal, volume 34, pages 495-498, 1942. Emmert, E. M.: Plant-Tissue Tests as a Guide to Fertilizer Treatment of Tomatoes, Kentucky Agricultural Experiment Station Bulletin 430, 1942. Hambidge, Gove, Editor: Hunger Signs in Crops, The American Society of Agronomy and the National Fertilizer Association, Washington, D. C, 1941. Merkle, F. G: Soil Testing, Operation, Interpretation, and Application, Pennsyl- vania Agricultural Experiment Station Bulletin 398, 1940. Morgan, M. F.: Chemical Soil Diagnosis by the Universal Soil Testing System, Connecticut Agricultural Experiment Station Bulletin 450, 1941. Peech, Michael, and English, Leah: Rapid Microchemical Soil Tests, Soil Science, volume 57, pages 167-195, 1944. Scarseth, George D. : Plant Tissue Testing i?i Diagnosis of the Nutritional Status of Growing Plants, Soil Science, volume 55, pages 113-120, 1943. Spurway, C. H. : Soil Fertility Control for Greenhouses, Michigan Agricultural Experiment Station Special Bulletin 325, 1943. Ulrich, Albert: Plant Analysis as a Diagnostic Procedure, Soil Science, volume 55, pages 101-112, 1943. Wolf, Benjamin: Rapid Soil Tests Furnish one of the Implements for Increasing Crop Yields, Better Crops with Plant Food, volume 29, pages 14-20, 47-49, 1945.