WTR AS A SOIL AMENDMENT TO REDUCE NONPOINT SOURCE POLLUTION FROM 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 main contributor to this 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. Four sites with soils having very high P test levels were selected in 1998/99 for field evaluation of utilizing water treatment residuals (WTR) to reduce these high levels. In MI, 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 MI. When Bray P1 levels exceed 300 lb P/acre, management practices in MI recommend not applying any additional manure and/or fertilizer P. Two sites were established for amendment with WTR in 1998 and two additional sites were established in 1999. The four sites had Bray P1 test levels of about 500, 1,200, 1,200, and 1,100 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 approximately 20 and 60 dry ton/acre. Soils were disked twice at each site following WTR application to mix the WTR with soil. Subsequent tillage for additional mixing and seedbed preparation prior to planting varied from site to site. Field corn was planted at each site in both years, and diagnostic leaf tissue samples and yield measurements of corn were taken during the 1998 and 1999 growing seasons. Soils were periodically sampled to evaluate changes in Bray P1 test levels with time. Alum WTR amendments to P-enriched soils in the field did not significantly decrease Bray P1 soil test levels one year after application. The decrease in Bray P1 soil test that was expected may be delayed, due to inadequate mixing of alum WTR and soil since the WTR were applied.