The tenets of 4R Nutrient Stewardship involve choosing the right source, applying the right rate, at the right time, in the right place for efficient crop use. Subscribing to these tenets carries with it a concern for the environment, the economic well-being of all both in agriculture and the public beyond. The best practices of managing N-P-K are based on these 4R principles and have proven to maximum the crops nutrient use while bringing home an ROI.
Managing nitrogen as system is the best way to maximize the dollars spent on fertilizer. The goal is to keep the nitrogen in the field and available for plant uptake when the crop needs nitrogen. The three strategies to managing nitrogen as a system are: using nitrogen stabilizers to prolong the amount of time nitrogen stays in the field, using spilt applications to increase the chance nitrogen is available when the crop needs it and to use a biological nitrogen source which will provide a low dose of nitrogen to the crop all season long.
Nitrogen stabilizers are easiest and most effective way to protect the investment a grower has made in nitrogen fertilizer. Using a nitrogen stabilizer in the spring is just as important as stabilizing in the fall. Warmer temperatures and increased rainfall limit the amount of time nitrogen can remain in the field.
Most synthetic fertilizers contain nitrogen as ammonium (NH4+) or a form of nitrogen that will be converted to ammonium. This is ideal because ammonium is easy for the crop to uptake, however, bacteria in the soil rapidly covert ammonium to nitrate (NO3-) which can easily be lost to leaching or denitrification. In a warm soil this can happen in as little as seven days. Nitrification inhibitors keep the applied nitrogen fertilizer in the ammonium form longer, preventing nitrogen from being lost to the environment and ensuring it is available for crop uptake.
Often, people think if they incorporate urea it is protected from volatilization losses, but this is not true. Regardless of whether the urea is incorporated, or left on the surface, urease inhibitors are needed to prevent urea from volatilization – when it turns into a gas and is lost to the environment.
Split applications of nitrogen are another crucial management step that prevents nitrogen loss and increases its availability for crop growth.
By splitting nitrogen fertilizer into multiple applications, we can limit the amount time the nitrogen is exposed to the environment and can be lost via denitrification, leaching and volatilization and deliver the fertilizer closer to the time when the crop will be uptake large amounts of N. Split applications are a risk management tool.
New products are emerging to help growers managing nitrogen by adding a biological source of nitrogen. These products generally include the application of living bacteria to the crop that can fix atmospheric nitrogen and provides a small amount of nitrogen to the crop throughout the growing season. The bacteria in these products differ but all work similarly to the rhizobia bacteria that fixes atmospheric nitrogen in legume crops. Envita™ and Utrisha™ N are two options offered through the GROWMARK/FS System.
The foundation for a successful crop begins in the soil below it. We must ensure that we have sufficient nutrients available in the soil for crop production and we do this by taking a soil test. Basing applications of P and K on grid sampling is the most accurate way to ensure that there is enough fertilizer available to support crop growth and limit unnecessary over-application of lime and fertilizer.
The best recommendations for P and K applications come from 2.5-acre grid samples pulled at the correct depth to match your local university recommendations. This information is then used to create a fertilizer and lime recommendation that ensures P and K fertility will not be your most limiting factor while utilizing any excess nutrients in the soil, if available, to reduce application rates.
We can further enhance the fertilizer recommendation by utilizing GPS referenced yield data to allow us to adjust the maintenance portion of the fertilizer recommendation to what has been removed from the field. As an example, an area of the field that yielded 170 bu will get maintenance for that yield where an area that may have only yielded 130 bu will get fertilized for that. This way we can maximize the return to the fertilizer dollars spent.
Lime applications to correct soil pH is a critical step for soil fertility. Soil pH needs to be between 6.0-7.0 to prevent nutrient tie-up and ensure that fertilizer applications can be used for crop growth (figure 1). Grid sampling is a good way to ensure we are only applying lime where needed. Variable rate applications of lime are important because if we apply lime, and increase our soil pH, where it is not necessary, we can tie up nutrients and make them unavailable to our crop (Figure 1).
Build-up applications of fertilizer are used to increase the levels of P & K to critical soil test levels. Building P & K to critical soil levels ensures that there is enough fertility in the soil to maximize yield. By building to critical levels, we make sure soil fertility will not be a yield limiting factor for the year. If P & K are below the critical test levels, fertility could be a yield limiting factor.