Prediction of Water‐Extractable Phosphorus Content of Soil Following a Phosphorus Addition

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.