Nitrogen measurement for variable-rate N management in maize

Numerous proximal and remote soil and plant-testing methods are available to guide variable-rate N management in maize production. These include preseason and late spring soil nitrate tests, late season stalk nitrate tests, handheld chlorophyll meter readings, remote sensing of soil and crop canopy properties, site-specific data from yield monitors and soil electrical conductivity maps. Field applicators have even been equipped to sense the N status of a crop and direct the application of any needed N on-the-go. However, the adoption rates of variable-rate N management are low worldwide despite the potential economic and environmental benefits of this practice and the availability of variable-rate application technology in key areas of maize production. A major obstacle is the finding that recommended N fertilizer rates based on yield goal are often poorly correlated with actual economically optimum N rates. Nitrogen response patterns are often field and season specific and can vary widely within the same field, further complicating adoption. Paired field comparisons of uniform- and variable-rate N management have revealed no consistent advantages for either strategy in yields achieved, whole-field N usage or N use efficiency by plants. Research with other agronomic crops also indicates that the scale for determining crop N need may be as low as 1 m2. An additional constraint is that growers implementing variable-rate N techniques often are required to install several adequately fertilized N reference strips in their fields that have proven to be both hybrid and season specific. In addition to these agronomic and logistical challenges, the maximum potential benefit of a variable N strategy vs. a conventional uniform-rate N strategy is typically between 12 and 37 USD($)/ha. When the cost of developing and executing the variable-rate strategy is considered, the practice shows a low economic return. In the future, innovations in N soil testing, better crop simulation models, and improved N sensing and application equipment may assist growers in capturing the benefits of site-specific N management in maize production.