Nitrogen Requirements of Conservation Tillage Systems

Introduction Conservation tillage has been one of the most rapidly adopted agricultural practices of the past 15 years (CTIC, 1983). The primary impetus for conservation tillage has been decreased soil erosion; fuel, labor, and machinery costs; and increased soil water storage and yields (USDA, 1975). Conservation tillage may be broad­ ly defined as tillage practices that reduce soil and water losses as compared with conventional tillage methods (Mannering and Fenster, 1983). The Soil Conservation Service more strictly defines conservation tillage as any system with 30 percent or greater of the previous crop's residue remaining on the soil surface following planting. Conservation tillage systems include no-till, ridge till, strip till, mulch tillage, reduced tillage, and minimum tillage. No-till is the most extreme example of conserva­ tion tillage, with the only primary soil disturbance created by coulters positioned ahead of planter units. Tillage practices can influence soil nutrient availabili­ ty. Conventionally tilled grain crops often yield greater than no-till treatments when the rate of nitrogen (N) fer­ tilizer recommended for conventional tillage is applied to both systems (Thomaset al., 1973; Bandel et al., 1975; Blevins et al., 1977). When slightly higher N rates are added, no-till yields may be equal or superior to con­ ventionally tilled crops. The increased N requirement for no-till may be due to several factors. Kitur et al. (1984) suggested that the large amount of surface residues associated with certain conservation tillage soils might result in considerable immobilization of surfaceapplied N. Conservation tillage soils may also be wetter and have larger continuous pores than conventionally tilled soils, enhancing leaching and denitrification losses (Thomas et al., 1973; Rice and Smith, 1982).Differences in fertilizer N requirements are usually most evident when comparing conventional and no-tillage systems. With increasing degrees of tillage in other reduced-tillage systems, however, differences will be less distinct. Fertilizer N placement often is an important con­ sideration in conservation tillage systems. Mengel et al. (1982) reported that subsurface banding of N resulted in greater no-till corn yields and suggested immobiliza­ tion and volatilization as possible reasons for the reduced effectiveness of surface-applied N in high-residue systems. Nitrogen source may also influence yields in conservation tillagc systems. Surface residue accumula­ tion is often associated with increased urease activity near the soil surface (Dick, 1984). Urea or urea-