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Relative Release and Retentiveness of Soil Phosphates¹

ABSTRACT

A kinetic equation is derived in terms of reaction products assuming a pseudo first order displacement of Soil-P by hydroxyl ion. Application of this equation to a Bridger silt loam by means of a nonequilibrium leaching system indicated at least two forms of soil P based on rate of release: (a) a rapidly released form which quickly diminishes with time, and (b) a more slowly released form which is relatively constant with time. An equation is also derived for the equilibrium adsorption of P on the soil surface from a soil-water system. Caribou, Bridger, Davidson, and Nibley soils were equilibrated with solutions of varying P concentrations at constant volume. The results were plotted using moles of P adsorbed as ordinate vs. the moles of P adsorbed divided by the equilibrium P solution concentration as abscissa. The data again indicated at least two forms of soil P, one tightly held and one relatively loosely held. At constant solution volume, equilibration of 1, 2, and 4 g. of Caribou soil with solutions of varying P concentration, indicated that the apparent dissociation constant for each site and the maximum amount of P that may be adsorbed per gram of soil by each site are both independent of the weight of soil used. The characterization of the soil P system by this equilibrium method is independent of soil/solution ratio in the range of 1:5 to 1:2000.

¹ Contribution from the Soil and Water Conservation Research Division, ARS, USDA, Beltsville, Md. Presented before Div. II, Soil Science Society of America, Aug. 5, 1958, at Lafayette, Ind.

² Soil Scientists.

Received for publication August 21, 1958. Accepted for publication October 24, 1958.