IDENTIFYING SITES VULNERABLE TO PHOSPHORUS LOSS IN AGRICULTURAL RUNOFF

The continual application of more phosphorus (P) in fertilizer and man ure to agricultural systems than removed in harvested crops can lead t o surface soil accumulation of P. As a result, soil P has become of en vironmental rather than agronomic concern in areas of intensive crop a nd livestock production where P enrichment of runoff can promote eutro phication. To target cost-effective remedial measures, sites vulnerabl e to P loss in runoff must be identified. Site identification by tradi tional field trials and computer simulations can be time consuming, co stly, and data intensive. The Lemunyon and Gilbert field index was use d to rank the vulnerability for P loss from 30 unfertilized and P-fert ilized, grassed, and cropped watersheds in the Southern Plains. Waters hed vulnerability to P loss in runoff was closely related (r(2) = 0.70 *) to actual losses measured over the last 16 yr (0.1-5 kg P ha(-1) y r(-1)). Vulnerability to P loss decreased in the order: conventionally tilled wheat (Triticum aestivum L.); no-till wheat; native grass; and set-aside grass. Also, erosive gully remediation by shaping, spriggin g with Midland bermudagrass [Cynodon dactylon (L.) Pers.] reduced vuln erability from medium to low. The P index is a valuable tool to identi fy P sources within a watershed that will require more intensive manag ement to minimize P loss in runoff while maintaining crop productivity .