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Availability of Essential Soil Elements—A Relative Matter¹

University of Wisconsin

ABSTRACT

The degree of availability of any one of the essential plant nutrients of the soil usually varies over a considerable range, depending on the form or forms in which it exists, and the condition of the soil, especially, as regards reaction and presence of excessive amounts of calcium carbonate. Phosphorus, for example, exists principally as calcium phosphate and basic iron phosphate. The former dissolves easily in weak acids and may be classed as readily available to plants, excepting when the soil contains an excessive amount of calcium carbonate (2 per cent or more). The latter form dissolves much more slowly in weak acids and is for the most part difficultly or slowly available. If, however, this latter form exists in large amounts, the rate of solution of phosphate from it may be rapid enough to supply the needs of a crop. The relation of these conditions to methods of determining the readily available phosphorus is discussed. The degree of availability or sufficiency of any particular element also varies with the climate, supply of other nutrients, and the kind of crop grown. Availability of the plant nutrients in the soil is, therefore, a relative matter. For both practical and scientific purposes, it is highly desirable to distinguish between the more readily available and less readily available portions of phosphorus and other nutrients of soils. The use of the terms "available" and "unavailable" in speaking of these two portions is not conducive to clear thinking and leads to confusion. In fact, this usage in the past, because of its erroneous implications, has been a barrier to progress and has at times placed soil analysis in disrepute. The use of terms like "readily available" and "difficultly available," in this connection, expresses the situation much more correctly and satisfactorily, and is a great help in any consideration of the subject.

¹ Contribution from the Department of Soils. Published with the permission of the Director of the Wisconsin Agricultural Experiment Station.

² Professor of Soils. The writer is indebted to Dr. L. A. Dean for the data in Tables I, II, and III, and Dr. H. H. Hull for the data in Table V.