Diagnostic tools for nitrogen management

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.

The nitrogen (N) status in soils can be quite dynamic, ever changing with soil moisture and temperature conditions. As soils warm in the spring, available N is released from crop residues, animal manures or soil organic matter by microbial decomposition. Once the N is converted to the nitrate form, it is at risk of loss. This year many areas of the state have experienced heavy and persistent rains. When soils become saturated by rain, N can be lost by leaching of nitrate with the downward moving water or by denitrification, conversion of nitrate to gaseous nitrous oxide. The longer the time between when N fertilizer is applied and when the crop is actively growing, the greater the risk for N loss. Timing N application to when the crop needs it is a good management practice. “How much N is available in the soil’ and “how much N should I sidedress” are two common questions asked by farmers. Coming up with a reasonable answer can be challenging given variable weather conditions. A soil N test can provide information about the available N status of the soil.

For vegetable crops, collect soil cores to a depth of 12 inches, mix the soil thoroughly and dry as quickly as possible before sending the soil to the testing lab. The test result will indicate how much N is available and will provide a guide for how much additional N, if any, is needed.

For actively growing vegetable crops, determination of the nitrate concentration in the petiole sap or in the dry petiole tissue can provide an indication of the N status of the crop. For growers that have nitrate meters, the petiole sap can be analyzed for a quick test. Regular monitoring to follow changes in the nitrate status can indicate when to apply N. Soil and plant testing labs can analyze dry petiole tissue for the nitrate content and provide interpretation of the results. For more information, contact the MSU Soil and Plant Nutrient Lab, 517-355-0218 or .(JavaScript must be enabled to view this email address).  

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