Does the crop or the soil indicate how to save nitrogen in maize production?: Reviewing the state of the art

High nitrogen (N) application rates are used by maize growers as an 'insurance', but may have an adverse effect on the environment. In this paper, the potential for nitrogen savings by the use of site-specific N fertilization rates is reviewed. Various tools for a more precise assessment of N requirements are available. Adjustments of N rates to the amounts of soil mineral N present shortly before planting can contribute to efficient N use, as can a deliberate delay of N dressings combined with indicator-based N supplementation after emergence. Conditions, under which these strategies are effective, are discussed. Post-emergence tests indicate whether a crop is likely to respond to sidedress N, but few tests quantify how much N should be sidedressed. Tissue tests are of less value for the support of decisions on N supplementation than indicators that are directly related to the soil or to the measurement of leaf and canopy greenness. Greenness and tissue tests are both unable to quantify excessive availability of N at early crop stages, as opposed to soil related indicators. Moreover, all crop tests generally need an on-site calibration with a reference plot. At later sampling dates, indicators can provide clues for the adjustment of N management in subsequent maize crops. The late stalk nitrate test and the post-harvest soil mineral N test appear to be the most valuable. Although indicators capable of identifying fields with an excessive N availability may save money, little attention has been paid to a cost-benefit analysis of indicator-based site-specific N management. (C) 2000 Elsevier Science B.V. All rights reserved.