Some common questions that we get asked about soil test results are, where is the nitrogen level on my report, and how do you recommend a nitrogen rate when you did not test the nitrogen level of the soil?
To begin, let’s explain why plant available nitrogen (N) is not part of our routine soil test packages. The simple answer is our climate. In the Midwest and northeastern United States, our “non-growing” seasons are when we receive much of our annual precipitation. That means that any excess applied N from a growing season probably will not carry over to the following growing season. It is likely to be lost to leaching and denitrification. So, we build our recommendations on the assumption that we are starting with little to no plant available N.
Since our recommend rate of N is not based on a soil test result, we use a projected yield goal. When harvested, a bushel of corn will remove about 0.67 pounds of N. To grow a 200-bushel corn crop we must supply a minimum of 134 pounds of N just to cover what is being removed. However, that crop is going to require more N than crop removal because we also need to grow the whole plant to grow the grain. A corn crop will take up about one pound of N in total per bushel grown, meaning that same 200-bushel crop really needs about 200 pounds of N supplied.
If you request recommendations from A&L Great Lakes for a 200-bushel crop, we will recommend 240 pounds. Most agronomists and growers will agree that this is an aggressive rate for 200-bushel corn, and it is. The reason that it is so high is that this is a starting point. We are assuming a worst-case scenario in which all the N is broadcast applied prior to planting. Unfortunately, N becomes more prone to loss the longer it stays in the soil before a crop can utilize it. A corn crop will take up about 70% of its N during the first half of the growing season and the remaining 30% will be taken up slowly all the way through physiological maturity. The likelihood of preplant application of N at a crop uptake only rate making it through the whole growing season is unlikely. The extra N supplied is to help ensure season-long availability. Fortunately, there are practices and tools that we can use to help lower the rate of N that needs to be applied.
The most common practice to lower the total amount of applied N is in-season and split applications. Whether it is sidedressing, high-clearance spreaders, or fertigation, the closer the N is applied to the time of crop uptake, the less likely it is for losses to occur. Most growers using a combination of starter N and a second application around V5 to V6, are comfortable using a rate around 1.1 pounds of N per bushel.
Another tool that can be used to reduce N application rates is the Estimated Nitrogen Release (ENR). In a good growing season, you can estimate that about 30 pounds of N will be mineralized from every 1% organic matter that is in your soil. If your soil has 3.5% organic matter, it has the potential to provide approximately 100 pounds. The problem with the natural mineralization of N is that it may not coincide with the timing that the crop needs it. So, it is not advisable to reduce the application rate by the entire 100 pounds, but 40 to 50 pounds maybe justifiable.
The Maximum Return to Nitrogen (MRTN) model is another option to use and is what the current Tri-State recommendations utilize. The idea of this model is to incorporate years of N response data from your area, N prices, and corn prices to predict the most profitable rate of N to apply.
The greatest challenge with recommending a N rate is that it is impossible to predict the weather and economy far enough into the future to know that the right decision is being made. The best way to go about making a recommendation is to use all of the tools that you have available and learn from your past practices.