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Tools Available for In-season Nitrogen Management Decision Making

By Charles White

Many farmers remembered last year’s excessively wet growing season and decided to split their nitrogen applications on corn this year, putting out only a portion of their total projected nitrogen (N) requirement at planting and waiting to apply the balance at sidedressing. Because of the numerous pathways for N loss, many of which are driven by wet weather, keeping your N fertilizer in the bag, bin, or tank until it is needed by the crop is one of the easiest ways to improve the efficiency of N fertilizer, especially in a wet spring like this one. Up until the V5 stage of corn development, N uptake by the corn is particularly low. At the V6 stage, corn has only taken up 15 to 20 lbs N/acre and by the V9 stage, N uptake reaches about 50 lbs N/acre. Therefore, depending on when you plan to sidedress, and if you expect to have significant N mineralization from soil organic matter, it may only be necessary to apply 25 to 50 lbs N/acre at planting. This year, with much of Pennsylvania already 2 to 8 inches over normal rainfall levels, those who split N applications will likely experience big improvements in N fertilizer efficiency.

For those who split their N applications, there are several in-season N availability assessments that can be used in the coming weeks to help decide on an appropriate sidedress N application rate. These assessments, the pre-sidedress soil nitrate test (PSNT) and the chlorophyll meter test, are designed to assess the amount of N that is becoming available from mineralization of soil organic matter. In fields with a frequent manure history or that are rotating out of a legume hay crop, significant N mineralization from organic matter can substantially offset the amount of N fertilizer that is required. These tests are not designed to detect how much N fertilizer that was applied at planting is still available in the soil profile, however. For those who applied their entire projected N fertilizer requirement at planting, assessing N availability and determining whether there is a need to sidedress additional N becomes more complicated and will be discussed at the end of the article.

The PSNT test involves taking a soil sample to 12” deep and sending the sample to a lab for analysis of the soil nitrate concentration. The test should be taken when the corn crop is 12” tall, or about the V5 stage. For normal planting dates, the sampling window usually occurs in mid-June. When relatively little N has been applied at planting (<50 to 60 lbs N/acre), the soil nitrate level at this growth stage is an indicator of the rate of N mineralization thus far in the growing season 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 (preferably before sending to the lab) by spreading the soil in a thin layer on a paper bag, paper plate, or newspaper with an electric fan blowing air over it to speed drying. Alternatively, moist soil samples can be kept refrigerated and you can drop them off in person at the soil testing lab. The point here is to prevent microbes in the soil from continuing to mineralize and nitrify N in the soil sample between when it is collected from the field and when it is analyzed in the lab. It is also important not to collect a soil sample immediately after a heavy rainstorm, since the rain can leach nitrate into the subsoil below the 12” sampling depth. Rather, wait several days after a heavy rainstorm so nitrate levels in the top 12” of 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.

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 hand-held 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 was applied at planting (< 50 to 60 lbs N/acre), the chlorophyll content of the corn leaves serves as 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 ."

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 lbs N/acre at planting, the nitrate accumulated from the fertilizer application will interfere with the PSNT’s detection of N mineralization and/or give you a false positive with the chlorophyll meter test (i.e., 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 profile 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.

To my knowledge, the only tools available to assess the losses of early applied N fertilizers, and whether there is a need to compensate for these losses with an additional sidedress N application, are computer models of the N cycle that take into account real-time, high resolution rainfall and temperature data for a location. Models such as Adapt-N and Encirca are now widely available and being increasingly used for N 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 N 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 in some cases may only be available through trained professionals that can help assure the integrity of the inputs and interpretation of the outputs.

I also think that the use of N computer models should still 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, I suggest setting 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, through which we can collectively improve upon and build greater confidence in our N management decision making.