For the second year in row, taking a pre-plant soil nitrate test this spring could pay big

By: Brad Carlson, Extension educator

The 2022 growing season was the second year in a row in which Minnesota experienced dry conditions across a majority of the state’s corn acres. The effects of drought extend beyond the growing season, with potential impacts on the following year’s crop. One area of interest is the potential for carryover nitrogen in the soil profile.

Most of Minnesota’s soils (coarse-textured sands excluded) mineralize significant amounts of nitrogen from soil organic matter throughout the summer. This nitrate is consumed by the growing crop until the crop reaches maturity. Typically, plant uptake of nitrate slows significantly around the first week of September and comes to a halt a few weeks after that. Mineralization of organic matter continues, however, with significant amounts of nitrate accumulating between early September and late October. Because the loss process of nitrate is water-based and requires saturation of the soil, a typical wet spring will cause this nitrate to be flushed via leaching or lost to the atmosphere through denitrification. It is for this reason that farmers typically cannot credit this “carryover” nitrogen when determining how much fertilizer to apply for the next year’s crop.

Dry conditions coming into spring mean a larger amount of water is necessary to saturate the soil, and leads to both less time saturated and less water percolating through the soil profile, which can result in less potential for nitrogen loss. These conditions provide an opportunity for farmers to test for soil nitrate and take the corresponding nitrogen credit, thereby reducing the nitrogen fertilizer rate they apply.

How to test your soil for residual N

U of M research conducted in the early 1990s established sampling protocol and soil test interpretations for use in crediting above-normal levels of soil nitrate. Samples should be taken to a depth of two feet due to the tendency of nitrate to move. Samples should be taken prior to any fertilizer or manure application, as it is difficult to know what form the nitrogen is in following application, and the test is for nitrates only. Fields with a long-term manure history and continuous corn provide the best potential for finding a credit. More detailed instructions for taking the pre-plant nitrate test (PPNT), as well as a chart to interpret results, can be found in the U of M’s corn fertilizer guidelines.

It should be noted that test results are reported in parts per million (ppm). Some testing laboratories may give results in pounds per acre but this should be avoided. A background level of soil nitrate of up to 6 parts per million (ppm) is typical, meaning no credit should be given until the soil test exceeds this point. Results in pounds per acre will include this value, making interpretation difficult.

Remember, while test results are accurate, the measured nitrate is still subject to loss if the soil becomes saturated after the soil samples were taken. For this reason, it is recommended that the test be taken as late as possible while still allowing for an overall N rate decision to be made. Because western Minnesota is less likely to experience loss conditions, the test if frequently taken there in the fall.

Fall soil test results

Data shared with the U of M by Minnesota Valley Testing Labs in New Ulm for tests taken this past fall (as well as for the fall of 2021) give a glimpse of the current situation. For the samples submitted in the fall of 2022, 78% had a measurable credit (compared to 72% in 2021). Nearly one-fourth had an N credit of 155 pounds (similar to 2021). This represents a potential savings of $100 per acre on N fertilizer, as well as resulting environmental quality benefits because the nitrate can be used by the crop instead of being lost. It is important to remember that this is not a random sample; these soil samples were submitted for testing because the fields fit the criteria indicating an N credit is likely.

The overall message is clear: there are many fields across Minnesota where significant fertilizer cost savings can be found via soil nitrate testing this spring. This could be another good year to take a pre-plant soil nitrate test.

Thank you to Minnesota Valley Testing Labs for sharing their data with us.

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