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Prediction of Water-Extractable Phosphorus Content of Soil Following a Phosphorus Addition¹

ABSTRACT

The effect of time on the water-extractable P content (P_X) of surface soil (0–10 cm) following a P addition (P_A) was investigated in the laboratory using 20 soils from 14 states. The soils were incubated with P equivalent to 0-, 25-, 50-, and 100-kg/ha additions and the water-extractable P content and soluble P concentrations of runoff from the soils determined at frequent intervals. Water-extractable P decreased in the first 2 weeks after P addition; following this P_X remained fairly constant. Water-extractable soil P content was linearly related to P_A at any given time after the addition. The slope of this relationship decreased logarithmically with time after a P addition. In the first 2 weeks after P addition, the slope of the relationship between P_X and P_A could be predicted from percent clay content or P sorption capacity of the soil. Using these simple relationships, an improved prediction of the amount of soluble P available for transport in surface runoff from fallow soil can be made from limited extractable soil P data.

Key Words: surface runoff • P desorption • P sorption capacity • water-quality modeling

¹ Contribution of the Department of Agronomy, Oklahoma Agric. Exp. Stn., Oklahoma State University, Stillwater, OK 74074. Published with approval of the director as paper no. 3947 of the Journal Series. In cooperation with an agreement with Southern Plains Watershed and Water Quality Laboratory, USDA-ARS, Durant, OK 74701. Agreement no. 58-7B30-8-22.

² Research Soil Chemist, Southern Plains Watershed and Water Quality Laboratory, USDA-ARS, P.O. Box 1430, Durant.

Received for publication April 13, 1981.