PHOSPHORUS IN RUNOFF FROM A FERTILE DAIRY PASTURE

Losses of phosphorus (P) from grazed pastures in runoff can significan tly reduce the water quality in streams draining agricultural catchmen ts. Many preventative strategies depend for their success on the assoc iation of the P with large soil particles (>0.45 mu m). We investigate d the forms of P in runoff from a dairy pasture of high fertility. A 3 .6-ha paddock at Darnum in West Gippsland, Victoria, was set up to mea sure runoff volume and rainfall, and to sample runoff water. Runoff fr om the site was restricted to 3 months in late winter and spring. Tota l runoff for 1994 of 660 m(3) had an average total P concentration of 5.2 mg/L. Runoff from a single storm system over 8 days in early Novem ber accounted for 56% of the total runoff volume and 69% of the P lost . Of the total P in the runoff, 93% passed through a filter with 0.45- mu m pores and 91% was reactive in an ascorbic acid-molybdate medium, without digestion; 89% of the P was both reactive in the ascorbic acid -molybdate medium and in the <0.45-mu m fraction. There was no apparen t relationship (P > 0.05) between P concentration and soil cover. In t he Darnum catchment, the major runoff events occurred when the soil wa s saturated. The low settling velocities of materials <0.45 mu m and t he amount of P moving in larger storms would suggest that buffer strip s and riparian zones are unlikely to decrease P losses substantially f rom dairy pastures in this area. Further, since erosion is unlikely to be the major process by which P is lost, control measures which simpl y limit erosion will be similarly ineffective at preventing P losses u nder the conditions that prevailed in this study.