IDENTIFYING SITES VULNERABLE TO PHOSPHORUS LOSS IN AGRICULTURAL RUNOFF

The continual application of more phosphorus (P) in fertilizer and manure to agricultural systems than removed in harvested crops can lead to surface soil accumulation of P. As a result, soil P has become of environmental rather than agronomic concern in areas of intensive crop and livestock production where P enrichment of runoff can promote eutrophication. To target cost-effective remedial measures, sites vulnerable to P loss in runoff must be identified. Site identification by traditional field trials and computer simulations can be time consuming, costly, and data intensive. The Lemunyon and Gilbert field index was used to rank the vulnerability for P loss from 30 unfertilized and P-fertilized, grassed, and cropped watersheds in the Southern Plains. Watershed 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) yr(-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, sprigging with Midland bermudagrass [Cynodon dactylon (L.) Pers.] reduced vulnerability from medium to low. The P index is a valuable tool to identify P sources within a watershed that will require more intensive management to minimize P loss in runoff while maintaining crop productivity.