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 QuestionEfficient 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 SummaryIn 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 DescriptionNitrogen 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 QuestionsWhat 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. 1). Fig. 1Relationship 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.