Surface Residue and Soil Moisture Affect Fertilizer Loss in Simulated Runoff on a Heavy Clay Soil

The potential for non-point-source pollution of surface waters from agricultural lands continues to be a concern. Our objective was to determine the effect of surface residue management and fertilizer application timing in regards to soil moisture conditions on nutrient losses in runoff. Studies were conducted using a rainfall simulator that applied 125 mm h-1 for 3 h to an Austin (Udorthentic Haplustoll) clay soil. Soil surface residue treatments were chisel tillage with no added corn (Zea mays L.) residue (CT-NAR), chisel tillage with added corn residue (CT-AR), and bermudagrass [Cynodon dactylon (L.) Pers.] sod (sod). Rainfall simulation was made following fertilizer (16–9–0 N–P–K) application to relatively dry (350 g kg-1 moisture) and relatively wet (500 g kg-1) soil on each of the residue treatments. Runoff samples collected from a 1-m2 area were analyzed for NO3 –N, NH 4 –N, and PO4 –P concentration and amount (kg ha-1). When fertilizer was applied to relatively dry soil, nutrient losses from both wet and dry runs combined were less than the losses with fertilizer applied to relatively wet soil. For wet runs, the CT-AR treatment reduced total PO4 –N loss fivefold 4 –P loss nearly sevenfold and NH compared with CT-NAR (1.2 vs. 8.0 kg PO4 –P ha-1; 3.9 vs. 18.9 kg ha-1 NH4–N), due to increases in time before initiation of runoff and lower nutrient concentrations in runoff. For our conditions, therefore, reduction in nutrient losses in runoff can be achieved by maintaining surface crop residue and applying N and P fertilizers to relatively dry soils. The largest loss of fertilizer nutrients occurred with sod treatments: losses of PO4 –P for the relatively wet soil were ≈ 41% of PO4 –P fertilizer applied (51.9 kg PO4 –P ha-1). This indicates that granular fertilizer application to pastures on heavy clay soils with vertic properties may make a significant contribution to non-point­ source pollution; careful management of granular fertilizer applica­ tions is thus called for, especially soil water content, when fertiliz­