Performance-Based Evaluations of Guidelines for Nitrogen Fertilizer Application after Animal Manure

Abstract

Nitrogen fertilizer needs for corn (Zea mays L.) in fields already treated with animal manure can be estimated by using general guidelines or soil testing for inorganic N. Although the soil-testing approach has been extensively evaluated for ability to predict yield responses to applied N under field conditions, the general-guideline approach has not been subjected to comparable performance-based evaluations. Fertilizer response trials were conducted in 205 manured fields to (i) compare the two approaches for ability to predict corn yield responses to fertilizer N applied after animal manure, (ii) identify reasons for differences in predictive ability, and (iii) explore the benefits of performance-based comparisons of the alternative approaches. Analyses showed that 34% of the observed variability in response could be explained by inorganic N concentrations whereas less than 5% of this variability could be explained by the general-guideline approach. The soil-testing approach, therefore, had greater ability to integrate the effects of all factors affecting yield responses across the range of conditions studied. Mean yield responses (0.55 Mg ha−1) were smaller than are usually detectable in individual trials, but they were great enough to prompt farmers to fertilize. Results of this study indicate that the most commonly accepted approach to estimating N fertilizer needs is less reliable than generally believed and, therefore, that superior approaches are likely to remain unrecognized unless the performance of the commonly accepted approach is objectively evaluated under realistic field conditions.