Soil testing and plant analysis to optimize nitrogen management in manured cornfields
Land application of animal manure is widely accepted as a method of recycling plant available nitrogen (N) within agricultural systems, but there is great need to improve the efficiency of this recycling. Soil testing and plant analysis are widely accepted as important tools for managing plant nutrients, but guidelines for manure management seldom employ these tools. The objective of this study was to develop guidelines for use of soil testing and plant analysis to optimize N management on manured cornfields. Data were collected in 205 on-farm trials where commercially prepared fertilizer N was applied at various rates to replicated and randomized plots within fields receiving animal manure as normally applied by the farmer. Grain yield responses were used to evaluate the ability of the tests to assess the sufficiency of N for plant growth and to estimate N fertilizer needs;Concentrations of nitrate in the surface 60-cm layer of soil when corn plants were 15 to 30 cm tall explained 37% of the observed variability in yield response. Concentrations of nitrate in cornstalks at the end of the season explained 41% of this variability. Amounts of manure-N applied, corn yield potential and other factors usually considered in manure management guidelines explained less than 10% of this variability;The soil and cornstalk tests were calibrated by defining probabilities of profitable response and mean net returns to fertilization for various prices for grain and fertilizer. Observed probabilities of profitable response for various categories considered ranged from 0 to 95%, and mean net returns to fertilization ranged from --1.17 to 1.61 Mg grain/ha. Good agreement between the tests was observed, so either of the calibrated tests can provide site-specific feedback that can be used to evaluate and improve N management on farms. The soil test offers the advantage of being able to guide in-season fertilization to correct for unexpected losses of manure-N that occur soon after application. The greatest benefit of using the tests, however, probably will result from identifying manure management practices that minimize losses of manure-N from soils and thereby make manure a more reliable source of N for crop production.