EFFECT OF ALUM WTR AMENDMENTS TO PHOSPHORUS-ENRICHED SOILS ON SOIL TEST PHOSPHORUS LEVELS IN A FIELD EXPERIMENT

Agricultural runoff and/or erosion can be a major contributor to nonpoint source pollution. Continued inputs of fertilizer and manure P in excess of crop requirements have led to a build-up of soil P levels which are of environmental, rather than agronomic concern, particularly in areas of intensive crop and livestock production. Six sites with soils having very high soil test P (STP) levels were established in 1998-2000 for field evaluation of utilizing alum water treatment residuals (WTR) to reduce high STP levels. In Michigan, the Bray-Kurtz P1 (Bray P1) extractant is used as part of the soil fertility test to determine plant-available P levels. When Bray P1 soil test levels reach 75-100 lb P/acre, the P2O5 fertilizer recommendations will usually be zero for most crops and yield levels grown in Michigan. When Bray P1 levels exceed 300 lb P/acre, manure management practices in Michigan recommend not applying any additional manure and/or fertilizer P. Two sites were established each year for amendment with alum WTR in the 1998, 1999 and 2000 growing seasons. The six sites had Bray P1 test levels of about 600, 1150, 1100, 1100, 620 and 360 lb P/acre. Alum WTR were applied to the 1998 sites at rates of 17 and 51 dry ton/acre and to the 1999 sites at rates of 20 and 60 dry ton/acre. In 2000, a different source of alum WTR was used at rates of 33 and 99 dry ton/acre. Soils were disked twice at each site following application to mix the WTR with soil. Subsequent tillage for additional mixing and seedbed preparation prior to planting varied from site to site. All six sites were rototilled in April/May, 2000 prior to planting to get a more thorough mixing of WTR and soil. Field corn (Zea mays L.) was planted at each site for most years, except soybeans (Glycine max L.) were planted at one site in 2000 and at one site in 2001. Diagnostic leaf tissue samples and yield measurements of corn and soybeans were taken during each growing season. Soils were periodically sampled to evaluate changes in Bray P1 STP levels with time. Bray P1 soil test levels declined at the two sites established in 1998, but alum WTR amendments did not contribute to this decrease. At the other four sites, some decline in Bray P1 soil test levels was observed, and alum WTR amendments significantly decreased STP levels compared to control soils. Results thus far suggest that the decrease in Bray P1 soil test that was expected from WTR amendments may be delayed due to inadequate mixing of alum WTR with the soil.