Corn Response to Long-Term Nitrogen and Phosphorus Fertilization

Accurate fertilizer N recommendations for irrigated continuous corn (Zea mays L.) production in the Great Plains are important for maximizing productivity and profit, while minimizing environmental impact of fertilizer use. The objectives of this study were to determine the effects of long-term N and P fertilization on grain yield and fertilizer recovery by irrigated continuous corn and to compare predicted N recommendations with actual N requirements. Six rates of N (0, 40, 80, 120, 160, and 200 lb/acre) and two rates of P (0 and 18 lb P/acre) in a factorial arrangement were applied annually from 1961 to 1991 on a Ulysses silt loam (fine silty, mixed, mesic Aridic Haplustoll) near Tribune, KS. Grain yields were greater with a combination of N and P than with either applied alone. The optimal N rate was about 160 lb N/acre and was relatively constant over time. Averaged over the last 10 yr, application of 160 lb N/acre increased grain yields 46% without P and 103% with P. The yield benefit from applied P has increased over time. With adequate N, P increased grain yields by 29% averaged over 31 yr and 48% over the last 10 yr. Phosphorus fertilization reduced grain moisture from 28 to 23%. Apparent fertilizer N recovery in the grain at 160 lb N/acre was twice as high with P as without P. A N recommendation model was used to compare estimated N requirements with actual N rates required to meet yield goals. Yields obtained with 160 lb N/acre were estimated to require 220 to 260 lb N/acre, or an overestimation of 30 to 60%. These data suggest that the crop factor used in the model may need to be reduced to improve accuracy of N recommendations. Research QuestionLong term fertilizer use is a common practice for irrigated continuous corn in the Great Plains. Accurately identifying the fertilizer N requirement is important for maximizing productivity and profit, while minimizing environmental impact of fertilizer use. This study quantified the effect of repeated fertilizer applications on optimal N rate and evaluated the accuracy of predicting N requirements using a N recommendation model. Literature SummarySeveral studies have shown that accurately identifying the optimal N rate is an important N management decision for maximizing fertilizer N recovery by the crop, minimizing residual fertilizer N after harvest, and reducing potential groundwater contamination. Establishing yield goals that are too high or not taking into account residual soil N can result in overestimating N requirements. Adequate amounts of other plant nutrients also are required to maximize fertilizer N recovery. Study DescriptionSix rates of N fertilizer (0, 40, 80, 120, 160, and 200 lb N/acre) and two rates of P (0 and 18 lb P/acre) were applied annually (1961–1991) to irrigated continuous corn on a Ulysses silt loam. Measurements included grain yield, grain N and P, grain moisture, and apparent fertilizer recovery. Applied QuestionWhat is the optimum N rate for irrigated continuous corn? In this long-term study, the optimum N rate was about 160 lb N/acre and was relatively constant across years (Table 1). Although fertilizer P was needed to maximize grain yield, it had little effect on optimal N rate. What effect does P have on fertilizer N efficiency and grain production? With optimal N, fertilizer P doubled apparent N fertilizer recovery. Fertilizer P decreased grain moisture and increased grain yields by 48% averaged over 10 yr when applied with optimal N. Do current N recommendation models accurately predict N requirements? No. In 2 yr of the study with average or above average yields, the model overpredicted N needs by 30 to 60% compared with observed optimal N rate. Table 1Grain yield response of corn to N and P. 1982–19911961–1991N rate−P+P−P+ Plb/acre------------bu/acre------------- 0 82 87 70 73 40109131107119 80115150121143120114166124159160120177131169200119173132166