Importance of Conducting a Soil Nitrate Test Prior To Sidedressing

February 21, 2024

Managing crop and soil nutrient levels should be at the forefront of grower’s minds, financially and environmentally. For corn, nitrogen (N) is the largest player. It is often applied several times throughout the year. It can be applied in the fall, spring (pre-plant), at planting, and sidedress. Nitrogen can come from commercial fertilizers, manure, or legume crops. Commercial fertilizers can be applied as a pre-plant, as a pop-up or 2 x 2 starter at planting, or after emergence. It is important to know when and how much fertilizer to apply to have a successful crop.

A Pre-Sidedress Nitrogen Test (PSNT) is used when the corn crop is around the V6 growth stage and about a week prior to the planned application of N. This test determines the level of nitrates in the soil at the time when corn N uptake is about to spike, and the plant begins a rapid growth phase. The test is best used when only a minimum of N is applied in the spring either as a starter fertilizer and/or when N is used as an herbicide carrier. The test helps determine if additional N should be applied. A PSNT does not measure ammonium N or organic N, it only measures nitrates, the form that is available to the plant. If the PSNT comes back at an acceptable level, there is usually no need for a sidedress application, saving the grower application and fertilizer costs. If the PSNT comes back low, it is a great tool to help the grower apply the correct amount of fertilizer for the desired yield goal. A test can also be done in the fall to see how much left-over nitrate is in the soil. If there is an excess amount left in the fall, this lets the grower know the rates may be able to be dialed back for the next crop. Excess N has the risk to leach out of the soil and potentially contaminate ground water and nearby bodies of water.

Every field planted in corn should be sampled. When taking a PSNT, about 15 representative soil cores should be collected. Do not take them all in one place. Samples should represent no more than a 20-acre block of a field. The cores need to be around 12 inches deep in the soil to get an accurate result. Mix the cores and send them off in a paper soil sample bag. Do not put the soil sample in a plastic sealable bag because of the potential for the sample to become anaerobic which can disturb the microbial action. It is also important not to sample the soil when it is saturated as this affects the microbes as well.

In summary, a PSNT is a key tool to help growers best manage the N levels of their soils helping them become more financially and environmentally successful.

Colby Rash
Channel Agronomist

Sources:

Soil nitrate test for corn. Michigan’s soil nitrate test for corn. Michigan State University Extension. https://www.canr.msu.edu/spnl/fact-sheets

Pre-sidedress soil nitrate test (PSNT). 2021. University of Maryland Extension. https://extension.umd.edu/resource/pre-sidedress-soil-nitrate-test-psnt/

Web sources verified 1/4/23. 1110_349103

Importance of Conducting a Soil Nitrate Test Prior To Sidedressing

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