ECONOMIC AND ENVIRONMENTAL IMPACTS OF LONG-TERM NITROGEN AND PHOSPHORUS FERTILIZATION

Nitrogen and P fertilizers are needed for optimum production and econo mic returns from irrigated continuous corn (Zea mays L.) However, exce ssive application of N must be avoided to minimize adverse environment al impacts. The objectives of this study were to determine: (i) the ec onomic optimum rate of N for irrigated continuous corn, (ii) how sensi tive optimal N rates were to corn and N prices as well as yield potent ial, and (iii) the relationship of optimal N rate to profile nitrate-N accumulation. Yield data from a long-term study were used to estimate quadratic production functions, with and without P, for corn yield as a function of N rate. Six rates of N (0, 40, 80, 120, 160, and 200 lb /acre) and two rates of P2O5 (0 and 40 lb/acre) were applied annually from 1961 to 1991 on a Ulysses silt loam (fine silty, mixed, mesic, Ar idic Haplustoll) near Tribune, KS. Without fertilizer P, soil P levels declined to deficient levels of about 7 ppm Bray 1-P. The economic op timal N rate, with P, was 159 lb/acre. Phosphorus fertilization increa sed the economic optimal N rate by about 15 lb/acre, while increasing maximum yields by almost 60 bu/acre, increasing maximum net revenue mo re than $130/acre, and decreasing minimum cost of production by $0.56/ bu. The economic optimal N rate was similar for low-, medium-, and hig h-yielding years and application of insurance N reduced profitability. The economic optimal N rate was relatively insensitive to changing N and corn prices. Profile nitrate-N content was relatively low and esse ntially the same when N was applied with P at the economic optimal N r ate or less. This indicates that applying N to maximize profit does no t cause nitrate accumulation or leaching concerns. However, accumulati on and movement of nitrate below the crop root zone increased when N w as applied in excess of the economic optimal rate or without adequate P fertilizer.