ALUM WTR AS AN AMENDMENT FOR PHOSPHORUS-ENRICHED SOILS

As point discharges of phosphorus (P) and other pollutants to surface waters from industrial and municipal wastewater systems have been reduced, nonpoint sources of P are now contributing a greater portion of P inputs into freshwater resources. Agricultural runoff and/or erosion can be a major contributor to this nonpoint source pollution. Continued inputs of fertilizer and manure P in excess of crop requirements have led to a buildup 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 P test levels were selected in 1998–2000 for field evaluation of utilizing alum water treatment residuals (WTR) to reduce these high 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, 1200, 1100, 1100, 800 and 500 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 in all three years, except one of the sites established in 1999 was planted to soybeans (Glycine max L.) in 2000. 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 test levels with time. Bray P1 soil test levels declined at the two sites established in 1998 from spring, 1998 to fall, 1999, but alum WTR amendments did not contribute to this decrease. At the 1999 sites, some decline in Bray P1 soil test levels was also observed during the six months of the 1999 growing season, and alum WTR did appear to contribute to some additional decrease at the high application rate. The decrease in Bray P1 soil test that was expected from WTR amendments may be delayed, due to inadequate mixing of alum WTR and/or the amount of active aluminum added by the WTR.