LABILE RESIDUAL FERTILIZER PHOSPHORUS IN CHERNOZEMIC SOILS. I. SOLUBILITY AND QUANTITY/INTENSITY STUDIES

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Abstract
Granular monoammonium phosphate (500 μg P/g soil) was applied in the field to three soils of an Oxbow catena. Changes in the inorganic P forms controlling P concentration or intensity (Ie) in soil solution during the ensuing 2½ yr were determined by equilibrium solubility product and related quantity/intensity analyses. In the Calcareous and Orthic control soils, Ie was controlled by impure hydroxyapatite. After P addition, it was controlled by dicalcium phosphate dihydrate and octocalcium phosphate. Marked changes in the quantity of labile P caused negligible changes in values for Ie. Therefore, in these and similar Chernozemic soils, Ie should adequately reflect the availability of residual P to plants. In the Gleysol after P addition, Ie was determined by adsorbed P or by the solubility of aluminum/iron-bound P forms. For this and similar Chernozemic soils, the quantity/intensity data indicated that estimates of the availability of residual P will require a measurement of its capacity to maintain Ie at the root–soil interface against depletion by plant uptake rather than a measurement of Ie alone.