Nitrogen Management Practices to Improve Crop Nitrogen Use Efficiency and Minimize Nitrogen Losses to the Environment
Basic recommended rates are determined based on your soil test report by looking at the planned crop and the expected yield for that crop. The amount of residual nitrogen in the soil must then be taken into account and subtracted from the recommendation. This includes previous manure applications and carryover N from previous legumes. Also, if fertilizer, such as a starter containing N, is applied regardless of manure applications, this N should also be taken into account. You need to account for these credits by subtracting them from the basic soil test recommendation (Figure 2). The resulting number will give you the rate you need to apply this year as fertilizer, manure, or other source of N.
Figure 2. Net crop nitrogen requirement.
- Do not apply nitrogen in excess of crop recommendations. Having an approved nutrient management plan can help you with this. See your soil test for the recommended rate and be sure to take into account the planned incorporation time, previous manure, and legumes.
- Manure application rates should be based on meeting the net crop need after all other sources of N either in the soil (legume N, manure residual N) or added N (starter fertilizer, N applied with herbicides) have been accounted for.
- Manure N availability to the crop is lower than the total amount of N in the manure. Thus, more total manure N must be applied to achieve the same results as would be needed using fertilizer to meet the same net crop requirement. However, manure N availability increases with optimum manure application management. The goal for optimum manure N management is to reduce the total N applied in manure to as close as possible to the amount that would be required as fertilizer.
- Best management such as applying manure in the spring, incorporating it immediately following application, and cover cropping will generally result in the highest manure N availability, less than two times the amount of fertilizer N that would be required to meet the net crop requirement. With good manure management, the total amount of manure N applied should be less than three times the fertilizer N requirement to meet the net crop requirement. Acceptable but less efficient manure N management may require more than three times the total manure N compared to fertilizer to meet the net crop requirement. See the Manure Nutrient Management section of the Penn State Agronomy Guide for information and instructions for making these critical calculations.
- Use the PSNT (pre-sidedress soil nitrate test) or chlorophyll meter to guide sidedress fertilizer nitrogen applications. The PSNT measures nitrate in the soil right before the highest amount of crop uptake. The chlorophyll meter test estimates the nitrogen status of growing corn by measuring the greenness of the leaves. Both of these in-season tests improve N recommendations significantly in most situations, particularly when manure is being used.
- Where appropriate, use new technologies such as on-the-go sensors and aerial photography that can provide useful information about the N status of crops, improve N recommendations, and enable variable-rate N application. Variable-rate N application has potential to improve crop yields and limit N environmental losses based on crop growth status and its interpretation for changing N rate application versus the traditional whole-field uniform-application-rate approach. Keep up on the latest technologies as they are developed and evaluated, and determine how they might fit into a program to improve N management on your farm.
SOURCE: PENN STATE EXTENSION