MANAGING AGRICULTURAL PHOSPHORUS FOR PROTECTION OF SURFACE WATERS - ISSUES AND OPTIONS

The accelerated eutrophication of most freshwaters is limited by P inp uts. Nonpoint sources of P in agricultural runoff now contribute a gre ater portion of freshwater inputs, due to easier identification and re cent control of point sources. Although P management is an integral pa rt of profitable agrisystems, continued inputs of fertilizer and manur e P in excess of crop requirements have led to a build-up of soil P le vels, which are of environmental rather than agronomic concern, partic ularly in areas of intensive crop and livestock production. Thus, the main issues facing the establishment of economically and environmental ly sound P management systems are the identification of soil P levels that are of environmental concern; targeting specific controls for dif ferent water quality objectives within watersheds; and balancing econo mic with environmental values. In developing effective options, we hav e brought together agricultural and limnological expertise to prioriti ze watershed management practices and remedial strategies to mitigate nonpoint-source impacts of agricultural P. Options include runoff and erosion control and P-source management, based on eutrophic rather tha n agronomic considerations. Current soil test P methods may screen soi ls on which the aquatic bioavailability of P should be estimated. Land owner options to more efficiently utilize manure P include basing appl ication rates on soil vulnerability to P loss in runoff, manure analys is, and programs encouraging manure movement to a greater hectareage. Targeting source areas may be achieved by use of indices to rank soil vulnerability to P loss in runoff and lake sensitivity to P inputs.