Use of Laboratory Extraction Data to Predict Losses of Dissolved Inorganic Phosphate in Surface Runoff and Tile Drainage

Abstract The mean concentration of dissolved inorganic phosphate (DIP) in each of several surface runoff events from established pasture was closely correlated with the amounts of inorganic P extracted by 0.1 M NaCl from the top 5 cm of soil prior to the event. For three of the four combinations of treatments evaluated (undrained, unfertilized; drained, unfertilized; and drained, fertilized) the linear relationships obtained were similar, despite large variations in the P concentrations observed. The mean DIP concentrations in surface runoff from the drained plots were consistently lower than those from the undrained plots. The relationship between the mean DIP concentrations of surface runoff and the amounts of extractable inorganic P for the undrained, fertilized plot was also linear but a greatly different slope was obtained. Soil extraction data appear to be useful in predicting mean DIP concentrations in surface runoff from reasonably homogeneous surface soils under pasture. More complex relationships were obtained between the amounts of extractable inorganic P in the soil at different depths in the profile and the mean concentrations and total loadings of DIP in tile drainage from a minicatchment adjacent to the surface runoff plots. Total DIP loadings were best correlated with the amounts of inorganic P extracted by 0.1 M NaCl from soil at 40–50 cm, a depth which corresponds to that of the mole channels.