We currently have more tools available to help with corn nitrogen management than we have ever had. Each of these tools has the potential to help us make better decisions, but none of these tools on their own should be viewed as a complete solution.
The Stage is Set
A pre-plant or pre-sidedress nitrate soil test is an excellent way to measure the amount of nitrogen available in the soil. The problem with a nitrate soil test, however, is that it is simply a snapshot in time of the soil nitrate level. In areas that receive excessive rainfall, we know that nitrate can be lost from the root zone through leaching or denitrification. Throughout the growing season, agronomists would love to know the current nitrate status of the soil, but conditions do not allow for sampling mud. Whole field sampling when corn is taller than 20 inches is also problematic.
Snapshot in Time
In-season sensors that measure the greenness of crop leaves can be used to help determine the need for sidedress nitrogen. One problem with this technology, though, is that the crop canopy must become full enough for sensors to detect reflection from vegetation, not from the soil surface. Greenness of the crop should also be thought of as a snapshot in time. It is not a measurement of soil nitrogen levels.
Some have proposed using plant tissue testing to determine the need for supplemental nitrogen. I believe that this could be a useful tool, but once again, sampling a whole field taller than 20 inches is a problem. We also know that calibrating tissue test nitrogen levels for different growth stages is inadequate to be able to deliver a sidedress recommendation for most stages of corn development.
Aiming for the Bull’s-Eye
Several predictive models have been developed to help with nitrogen management. I am thankful to have nitrogen models available, but I have come to believe that it is not realistic to expect that the current models can possibly handle all of the variables that occur across the landscape. Corn on the same soil type could have different tillage systems among various farms. One farm might use cover crops while another does not. Dairy manure characteristics can vary widely from one farm to the next depending on the bedding source and manure system design. Does the manure system consist of a single storage pit, or are there multiple pits used for flushing barns? Does the farm have a methane digester? These are just a few of the countless factors that may vary among farms, and they make it impossible for our current models to adequately predict soil nitrogen status. A model is much more likely to be able to account for the variables found within a single farm if the model is custom fit to conditions on that farm.
Fit To the Farm
We need an adaptive nitrogen prediction model that becomes customized to an individual farm over time. We need to be able to plug in any of the nitrogen management measurements at different corn growth stages so that the model can be self-adjusted over time. The goal would be to customize the model for the set of variables that exist on that one farm. There are occasions when agronomists will want to use a soil nitrate test and other times when a different measurement tool is more practical.
Nitrogen predictive models should be “plug and play” ready so that all of the available nitrogen status tools can be used to adjust the model itself. I believe that this approach will bring us one step closer to precision nitrogen management. When it comes to the multiple choices of nitrogen management tools, the correct answer is “all of the above.”