Economic and Environmental Impacts of Long‐Term Nitrogen and Phosphorus Fertilization

Nitrogen and P fertilizers are needed for optimum production and economic returns from irrigated continuous corn (Zea mays L.) However, excessive application of N must be avoided to minimize adverse environmental impacts. The objectives of this study were to determine: (i) the economic optimum rate of N for irrigated continuous corn, (ii) how sensitive optimal N rates were to corn and N prices as well as yield potential, 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, Aridic Haplustoll) near Tribune, KS. Without fertilizer P, soil P levels declined to deficient levels of about 7 ppm Bray 1‐P. The economic optimal N rate, with P, was 159 lb/acre. Phosphorus fertilization increased the economic optimal N rate by about 15 lb/acre, while increasing maximum yields by almost 60 bu/acre, increasing maximum net revenue more than $130/acre, and decreasing minimum cost of production by $0.56/bu. The economic optimal N rate was similar for low‐, medium‐, and high‐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 essentially the same when N was applied with P at the economic optimal N rate or less. This indicates that applying N to maximize profit does not cause nitrate accumulation or leaching concerns. However, accumulation and movement of nitrate below the crop root zone increased when N was applied in excess of the economic optimal rate or without adequate P fertilizer. Research Question Efficient use of fertilizer N is important for maximizing profit and minimizing environmental risk from fertilizer derived nitrate‐N. Although many soil, crop, and environment factors influence the quantity of soil profile nitrate‐N that could be transported below the root zone, accurately estimating fertilizer N requirements is the most important factor. The objectives of the study were to evaluate the relationship between economic optimum N rate and residual fertilizer N accumulation in the soil profile and to determine the sensitivity of optimal N rate to changing N and corn prices as well as yield potential. Literature Summary In the Great Plains, N recommendation models generally are based on yield goal and preplant nitrate‐N content in the soil profile. When yield goals are overestimated or profile nitrate‐N is underestimated, the. predicted N rate is greater than that required for optimum grain yield, which reduces economic returns and can dramatically increase residual fertilizer N in the soil profile. Several studies have reported minimal carryover of residual fertilizer N when the application rate was nearly equivalent to the requirement for optimal yield. Study Description Nitrogen and P fertilizers were applied annually from 1961 to 1991 to irrigated continuous corn in western Kansas. Six rates of N fertilizer (0, 40, 80, 120,160, and 200 lb/acre) and two rates of P2O5 (0 and 40 lb/acre) were arranged in a randomized complete block design with five replications. Grain yield response to N and P was measured each year, and soil profile nitrate‐N content (0 to 10 ft depth) was determined after harvest in 1990. A quadratic production function was estimated to describe grain yield response to applied N and P and to determine N rates at which yields and net returns were maximized and production costs per bushel were minimized. Applied Questions What is the economic optimum N rate for irrigated continuous corn in the central High Plains? In this long‐term study, the economic optimum N rate was 159 lb/acre. Although fertilizer P increased net revenue by $130/acre, it had little effect on optimal N rate. Does the economic optimum N rate vary from year to year because of differences in yield potential? No, the economic optimal N rate was essentially the same for low‐, medium‐, and high‐yielding years. Does the economic optimal N rate vary as fertilizer N and corn prices change? No, the economic optimal N rate was relatively insensitive to changing fertilizer N and corn prices. Does N fertilization at the most profitable rate cause nitrate‐N accumulation and potential leaching problems? No. After 30 yr of N application with P, nitrate‐N levels were very low when N was applied at the economic optimal rate with P (Fig. ). Relationship of economic optimal N rate to profile (10 ft) nitrate‐N content after 30 yr of N fertilization with P. Vertical line indicates economic optimal N rate.