Submitted to: American Society of Agronomy Monograph Series
Publication Type: Book / Chapter
Publication Acceptance Date: 7/26/2007
Publication Date: 6/20/2008
Citation: Meisinger, J.J., Schepers, J.S., Raun, W.R. 2008. Crop Nitrogen Requirement and Fertilization. American Society of Agronomy Monograph Series. 14:563-612. Interpretive Summary: Estimating crop nitrogen requirements and fertilizer nitrogen needs is a major factor in designing nitrogen management plans that maintain yields and protect environmental quality. This manuscript provides a detailed review of the principles under-girding nitrogen recommendations, which are the nitrogen mass-balance and the concept of soil-plant nitrogen resiliency. The mass-balance principle provide the basis for making pre-planting fertilizer nitrogen recommendations for cereal crops by accounting for crop nitrogen needs and local nitrogen sources, such as manure or legume residues. Soil-plant nitrogen resiliency forecasts an increased supply of available nitrogen in high yielding years, and contributes to the success of economic based recommendation systems. 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 nitrogen recommendations, but their manual deployment limits them to small acreages or areas requiring critical nitrogen management. The most recent approaches utilize real-time near infrared-visible reflectance sensors that compare crop nitrogen status to a nearby nitrogen sufficient crop, followed by an application of a variable nitrogen rate based on local crop-nitrogen stress. By combining traditional pre-plant nitrogen recommendations with some form of within-season sensing, crop advisors and nutrient managers should be able to develop nitrogen management plans that will increase nitrogen use efficiencies, improve profitability, and reduce nitrogen losses to the environment.
Technical Abstract: Estimating crop nitrogen (N) requirements and fertilizer N needs is the primary factor for designing N management plans that maintain yields and protect environmental quality. This manuscript reviews the principles under-girding N recommendations, which are the N mass-balance and the concept of soil-plant N resiliency. The mass balance principle applies to fertilizer-N recommendations by estimating crop N needs and adjusting for local N sources. The second principle, N resiliency, describes the capacity of a soil-plant system to vary plant available N with the growing conditions, which is illustrated by an increased supply of available N in high yielding years. If reliable yield estimates can be made prior to planting, or if soil-plant N resiliency allows use of average yields, the N mass-balance approach utilizing the difference between crop N need and soil N mineralization, along with N credits from soil nitrate, legume credits for soybeans or forage legumes, and manure credits is an accepted approach that utilizes site-specific data and has a clear educational value to the producer. However, this method requires a significant amount of local N data (soil nitrate, manure analyses, etc.), can be undermined by overly optimistic yield estimates, and traditionally does not contain economics, although economic adjustment factors can be added. The conventional economic approaches are easy to use and avoid problems with estimating yields. However, they have limited ability to predict specific fertilizer-N needs for a given site because they are based on response functions aggregated over large geographic areas. Economic approaches rely heavily on soil-plant N resiliency and require a continual commitment to measuring crop fertilizer-N responses for the current varieties and the common agronomic practices of an area. In practice, the traditional economic approach is usually combined with mass-balance N credits to improve recommendations for specific fields or cropping systems. Recent developments have focused on remote sensing and geo-referenced tools that offer the prospect for improving traditional pre-plant approaches. The use of manual within-season techniques, like the pre-sidedress soil nitrate test and the leaf chlorophyll meter, are useful for small acreages or areas requiring critical N management, but they are too laborious for large areas. Real-time mid-season crop sensors based on reflectance of near-infrared and visible light coupled with variable-rate N applicators have potential advantages over pre-plant approaches because they can utilize sub-meter resolution to monitor crop N status and estimate N response. These approaches are conducive to variable-rate N application strategies that are commercially available, however, these approaches require algorithms to interpret the crop N status and estimate fertilizer-N needs. Reliable algorithms have been developed for wheat and algorithms for other crops are being developed, but challenges remain such as the narrow window for N application, dependance on high-clearance equipment, and the risk of reduced yields if N deficiencies are not corrected early. 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, improve profitability, and reduce N losses to the environment.