Diagnostic tests for managing nitrogen
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Nitrogen (N) is a key nutrient for production of quality vegetable crops, and many vegetables are side- or topdressed with nitrogen during the growing season. Mid-season applications help match the supply of nitrogen with the requirement of the crop, and also minimizes the potential for loss of nitrogen by either leaching or denitrification. Depending on management practices, soil organic matter content, soil temperature and moisture conditions, the amount of nitrogen becoming available (mineralized) from the soil may vary from year to year and field to field. A soil nitrate test or a petiole sap nitrate test can provide information about the available nitrogen status of the soil. Nitrogen that becomes available in the soil due to microbial activity is nitrogen that does not need to be applied. Collecting soil samples to 10 to 12 inches deep prior to sidedressing and having the soil analyzed for the nitrate and ammonium nitrogen content, provides an indication of the available nitrogen status of the soil and provides a guide for how much additional nitrogen, if any, should be applied. Most soil testing labs, including the MSU Soil and Plant Nutrient Lab, will run the soil nitrogen test.
The total nitrogen concentration in a plant is a good indicator of nitrogen available in the soil. The nitrate concentration in the petiole correlates well with the total nitrogen content, and is the basis of a quick test to determine the nitrogen status of the crop. The petiole sap nitrate test can be used in two ways. The best is to monitor the petiole sap nitrate concentration on a regular basis (every seven to ten days) and follow the change. If there is a large decrease from one time to the next that is an indicator it is time to apply more nitrogen. There are also general guidelines for various crops that test values can be compared with to use as an indicator of whether or not nitrogen needs to be applied. These guidelines will be published in the next vegetable newsletter.