In February’s issue I touched on the use of the late spring nitrate test (LSNT) to help optimize the use of nitrogen fertilizer. At that time we were concerned with high nitrogen prices and the potential of limited nitrogen sources. We didn’t know of the other difficulties some would endure with wet and/or flooded soils, delayed planting and heavy rainfall events that further reduce N applications and may have increased N loss. Now that the corn crop is in the ground and most fields are up and growing, it is time to implement the LSNT procedure. Proper sampling procedures and interpretation of the results is essential to make the LSNT effective on your operation.

• When to sample: Soil sampling should occur when corn is from 6 to 12 inches tall
• Sample area: Samples should be collected within several test areas that are 1 to 10 acres in size
• Depth of sampling: Samples should be taken to a depth of 12 inches.
• Number of cores per samples: Samples should be comprised of 16 to 24 cores.
• Handling and shipping: Refrigerate or air dry the sample if it will not be at the lab within two days.
• Soil Analysis: The LSNT is based on concentrations of nitrate-N in the soil sample. Most soil testing labs can perform this analysis.

Often times the most difficult procedure with the LSNT is in interpreting the test results. Interpretations vary with cropping and manure history as well as with weather conditions prior to and after sampling. Table 1 should be used when corn is preceded by alfalfa and/or has a manure history in two of the previous four years.

When corn follows soybeans or corn, the critical N concentration is typically set at 25 pmm. With the situation across most areas this spring where excessive rainfall has occurred, the critical concentration should be lowered to 20 ppm. Excessive soil moisture generally moves nitrate deeper into the soil profile, often times below the sampling zone. The sample results may not accurately predict the total nitrate in the rooting zone.

Estimate the additional N needs by subtracting the soil-test nitrate-N concentration from the critical concentration, either 20 or 25. Multiply the difference by 8, the result is the required N to apply to the field. A factor of 8 is used because studies have shown that it usually takes 8 lb of N/acre to increase soil-test nitrate-N by 1 pmm.

Using the LSNT can be a very useful tool to help predict current nitrogen status in the soil. Yet results may be variable and personal discretion should be used when results appear to be far from what would be expected. Increased use and field history will increase the reliability of the LSNT on individual fields.