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Test your soil with the pre-sidedress nitrate test or chlorophyll meter test.

June 17, 2019

5 Min Read
Aerial view of tractor applying liquid nitrogen fertilizer to corn field
SIDEDRESS DECISIONS: Now is the time to think about when to sidedress corn. The pre-sidedress nitrate test, chlorophyll meter test and computer modeling can help you decide when the time is best to apply more nitrogen.BanksPhotos/Getty Images

Most farmers remember last year’s excessively wet growing season and, as a result, have decided to split their nitrogen applications on corn this year, putting only a portion of their total projected nitrogen requirement at planting and waiting to apply the balance at sidedress.

Because of the many ways nitrogen can be lost, many of which are driven by wet weather, keeping your fertilizer in the bag, bin or tank until it is needed is one of the easiest ways to improve your nitrogen use efficiency, especially in a wet spring like this one.

Up until the V5 stage of corn development, nitrogen uptake is relatively low. At the V6 stage, corn has only taken up 15 to 20 pounds of N per acre and by the V9 stage N uptake reaches about 50 pounds an acre.

For those who split their nitrogen applications this year, there are several in-season tools that can be used to help decide on an appropriate sidedress application rate:

1. PSNT test. The pre-sidedress nitrate test involves taking a soil sample down to 12 inches deep and sending it to a lab for analysis of the soil nitrate concentration. The test should be taken when corn is 12 inches tall, or at about the V5 stage.

For normal planting dates, the sampling window usually occurs in mid-June. When relatively little nitrogen has been applied at planting — between 50 and 60 pounds N per acre — the soil nitrate level at this growth stage is an indicator of the rate of N mineralization thus far, and this value has been calibrated to predict future N mineralization throughout the growing season. The level of N mineralization predicted by the test is then used to adjust the sidedress N requirement.

Soil samples collected for the PSNT should be dried immediately after collection by spreading the soil in a thin layer on a paper bag, paper plate or newspaper with an electric fan blowing air to speed drying. Moist soil samples can be kept refrigerated and then dropped off at the soil testing lab.

The point here is to prevent microbes in the soil from continuing to mineralize and nitrify N in the sample from when it is collected to when it is analyzed in the lab. It is also important to not collect a soil sample immediately after a heavy rainstorm since the rain can leach nitrate into the subsoil below the 12-inch sampling depth. Instead, wait several days after a heavy rainstorm so nitrate levels in the top 12 inches of the soil can recover.

The PSNT fact sheet, "Pre-sidedress Soil Nitrate Test for Corn," has more information about the test and a formula for calculating a recommended sidedress N application rate based on the field history, yield goal and the PSNT result.

2. Chlorophyll meter test. Another option for assessing N mineralization in the soil and determining how much N to sidedress is the chlorophyll meter test.

With this test, a handheld sensor clips onto the leaf blade of a corn plant and measures the chlorophyll content of the leaf. Chlorophyll is the nitrogen-rich molecule in a plant leaf that is essential for photosynthesis and gives the leaf its green color.

When relatively little N fertilizer is applied at planting, the chlorophyll content of the corn leaves is an indicator of soil N mineralization rates. Higher N mineralization rates, as detected by greater chlorophyll content of the corn leaf, will lead to lower sidedress N recommendations.

More details about this test, along with the formulas for calculating a sidedress N recommendation, are available in the chlorophyll meter factsheet "The Early Season Chlorophyll Meter Test for Corn."

3. Computer modeling. The only tools available to assess the losses of early applied N fertilizers and whether there is a need to compensate with an additional sidedress N application are computer models of the N cycle that consider real-time, high-resolution rainfall and temperature data for a location.

Models such as Adapt-N and Encirca are widely available and being used for nitrogen management decision-making. These computer models are a compilation of the best available science of how the N cycle operates and the expert judgement of model developers on how to represent these cycling processes as algorithms in the computer code.

Ultimately, the success of these models also depends on having accurate inputs, including soil profile characteristics and previous N management practices, and being able to interpret the outputs correctly. Because of this, N modeling services may only be available through trained professionals who can help ensure the integrity of the inputs and interpretation of the outputs.

The use of N computer models should be considered experimental. While there is much promise in these tools, there is still much to be learned and improved upon.

If you are planning to use computer N models in your decision-making this year, set up a simple on-farm experiment to compare the model-suggested sidedress rate with your typical practice. Experimenting with these and other N management tools is an important part of an adaptive management process.

As described earlier, these tests are designed to be used when a minimal amount of N is applied at planting. If you applied more than 50 to 60 pounds per acre at planting, the nitrate accumulated from the fertilizer application will interfere with the PSNT’s detection of N mineralization, and it could give you a false positive with the chlorophyll meter test (the test says you will have sufficient N available when you really won’t). In this case, there are very few field assessments, short of taking deep cores of the soil to measure nitrate and ammonium levels, that will help you determine if the large quantity of N you applied at planting is still available within the rooting depth of the crop.

Source: Penn State University, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.

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