Crop Nitrogen Requirement and Fertilization.

Estimating crop N requirements and fertilizer N needs is a major factor in designing N management plans that maintain yields and protect environmental quality. This chapter provides a detailed review of the principles under-girding N recommendations, which are the N mass-balance, the economic optimum, and the concept of soil-plant N resiliency. The mass-balance principle provides the basis for making pre-planting fertilizer N recommendations for cereals by accounting for crop N needs and local N sources, such as manure or legume residues. The Soil-plant N resiliency hypothesis forecasts increased supplies of available N in high-yielding years, and contributes to the success of economic based systems. Economic approaches group similarly responding sites and estimate optimum N rates based on costs of N fertilizers relative to grain prices. These pre-plant approaches are not able to adjust for within season weather and are only site-specific in a broad sense, being adjusted for field history and/or soil N resources. The use of manual within-season techniques, like the pre-sidedress soil nitrate test and the leaf chlorophyll meter, have proven useful for improving pre-plant recommendations, but their manual deployment limits them to small acreages or areas requiring critical N management. The most recent approaches utilize real-time near infrared-visible reflectance sensors that compare crop N status to a nearby N sufficient crop, followed by an application of a variable N rate based on local crop-N stress. They include within season weather conditions up to the time of sampling, and can be compared to a range of within-field N rates called “ramped N applications”. By combining traditional pre-plant N recommendations with some form of within-season sensing, crop advisors and nutrient managers should be able to develop N management plans that will increase N use efficiencies, improved profitability, and reduced N losses to the environment.