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 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 different elements in the plant tissue below which growth is retarded must be reasonably constant under different 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 effect 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 are now made on a purely experimental basis. Again, as in soil tests, there is a complete lack of standardization with respect 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.