Soil test phosphorus and clay content effects on runoff water quality

Dissolved phosphorus (P) in runoff increases with an increase in soil test P, but slopes found for this relationship vary by an order of magnitude. A difference in clay content contributes to this variation. Experiments were conducted on Ultisols of 5 and 32% clay by incorporating poultry (Gallus gallus domesticus) litter and triple superphosphate (TSP) to create a wide range in Mehlich 3-extractable phosphorus (M3P). Mehlich 3 P increased similarly for the two sources with rate of P applied, but the increase was greater as clay content decreased. Total P in runoff was measured and it was related primarily to the quantity of sediment. Dissolved P (mg L-1) was measured with simulated rainfall and it increased with an increase in M3P (mg dm(-3) in the surface 15-20 cm of soil). The slope of dissolved P versus M3P was 0.0040 for the 5% clay soil and 0.0014 for the 32% clay soil. With these relationships, M3P would need to be 250 mg dm-3 at 5% clay and 714 mg dm(-3) at 32% clay to result in 1.0 mg L-1 dissolved P in runoff, Dissolved P in samples of runoff collected from natural rainfall with automatic samplers was substantially less than that with simulated rainfall. Thus, there was a marked difference due to method of measurement as well as the difference noted due to clay content. Both factors should be considered if attempting to predict the level of dissolved P in runoff from soil test data.