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"Root Solubility" of the Essential Elements in the Soil as an Indication of Availability¹

ABSTRACT

Chemical methods proposed for determining the availability of the essential elements in the soil vary all of the way from leaching with distilled water to boiling with strong acids and biological methods, from the growth of microörganisms in small culture dishes to complete field experiments. With chemical methods the present tendency is predominantly toward the use of dilute acids and salt solutions as extracting agents.

Such methods furnish valuable information regarding the capacities of soils to supply plant nutrients. However, they indicate true availability only insofar as they are able to duplicate the effects of growing plants. The processes by which plant roots are active in bringing into solution and absorbing the essential elements in the soil are complex and, as yet, little understood. It appears highly improbable that any chemical method can be made to duplicate such effects. In addition to their solvent action, the roots of growing plants exercise considerable selectivity in the absorption of ions, thus materially altering the equilibriums set up in the soil.

During recent years the Neubauer method has been used extensively in availability studies and found to possess considerable merit. This procedure is based upon the extraction of a relatively small amount of soil by a large number of rye seedlings. Results indicate the amounts of the essential elements which can be taken up from the soil when contacted by the roots of plants growing under conditions of an inadequate nutrient supply. This appears to offer a logical basis for defining and determining availability.

Data are presented to show considerable differences not only between the results of the Neubauer method and chemical tests but also between results of the various chemical tests, themselves. This is especially true for the phosphorus values.

¹ Contribution from the State Chemist Department, Purdue University Agricultural Experiment Station, Lafayette, Ind., under a fellowship supported by the American Potash Institute, Inc. Presented as a part of the symposium on "What Constitutes Availability of the Essential Elements in the Soil" at the annual meeting of the Society held in Washington, D. C., November 18, 1936.