SOIL TEST PHOSPHORUS SAMPLING STRATEGIES TO OPTIMIZE MANURE APPLICATION IN AGRICULTURAL FIELDS 1 2 3 4 5 6

ABSTRACT. Phosphorus (P) is necessary for plant productivity, but excessive P can have a negative environmental impact. Manure application to fields can raise the possibility of P contamination of water bodies. That P contamination potential is related to management, especially the bioavailable soil test P (STP) level. Environmental indices to guide applications of animal waste are based, in large part, on STP. As STP rises, the manure rate should decrease. Field-average rates may result in excessive manuring of environmentally sensitive areas, but site-specific application depends on spatial sampling strategy/quality. In-field variation must be well described. Two fields under no-tillage management were chosen, and one (112) had been fertilized, while the other (950) had been both fertilized and manured. Three risk levels (low, medium, high) were defined according to a laboratory STP versus water-soluble P relationship. As sample grid size increased, both areal extent and location of risk levels changed. In 112, doubling grid size increased low risk area and decreased medium risk area. In 950, doubling grid size increased both low and high risk area at the expense of medium risk area. The number and location of samples influenced future manure management. Sampling research is needed for greater environmental benefits to site-specific manure application