By Jared Spackman
Wet conditions last fall and this spring have limited the number of acres that received nitrogen in preparation for the upcoming growing season. This creates the challenging dilemma of whether to delay planting in order to get fertilizer down. Most Minnesota soils can supply sufficient nitrogen to meet early corn growth without a yield reduction. The best bet is to plant corn on time to optimize yield potential and come back later to apply nitrogen.
Which fields should I focus on early fertilization?
Minnesota typically receives about a third of its annual precipitation in the months of April through June. Early applications of nitrogen may be subject to significant losses, especially if it converts to the nitrate form. During this time, corn demand for nitrogen is low, with only about 10% of the total N taken up by mid-June. Fields that are on hilltops or less prone to water-saturated conditions may be better candidates for early fertilization. Low-lying wet fields and fields with substantial drainage, because of natural conditions or intense tile-drainage, may benefit from later applications, when potential for nitrogen loss is less. Additionally, full-rate early applications of nitrogen should be avoided on sandy-textured soils in favor of later sidedress applications.
Research has shown that corn following soybean has low nitrogen needs early in the season, allowing for later fertilizer applications. Corn following corn is a different story. In this situation, supplying some nitrogen (around 40 lbs. N/acre) is important to help the crop until it is possible to apply the full amount of nitrogen needed.
Which products should I consider for early fertilization?
Fertilizer applications in April and May are typically more susceptible to nitrogen loss because of excess water and the corn crop being small with low nitrogen requirements. Ammonium binds to soil particles while nitrate moves with soil water. Fertilizer products that keep nitrogen in the ammonium form longer (such as anhydrous ammonia, polymer-coated urea or blends of urea and polymer-coated urea) are less subject to loss than those that transform rapidly to nitrate. That said, the benefit of a nitrification inhibitor in the spring has been inconsistent. Only under excessively wet springs are nitrification inhibitors more likely to provide a benefit, yet even then the response has not always been consistent.
Urea ammonium nitrate (UAN) is an excellent source for sidedress application time, but because 25% of the nitrogen is nitrate, there is considerable potential for loss with early applications. Also, keep in mind that urea-containing fertilizers, such as urea or UAN, left on a warm, moist soil surface are subject to substantial volatilization losses within a few days. If possible, you should plan to incorporate your nitrogen fertilizer with tillage down at least a couple of inches or with at least 0.25 inch of rainfall. If this is not possible, a urease inhibitor can extend the window for fertilizer incorporation.
How late can I wait to sidedress?
We often see little or no difference between a full rate of pre-plant nitrogen compared to a split application on fine-textured soils. However, if there is no time to apply all the nitrogen without delaying planting, a small amount of nitrogen applied as starter is all you need to carry the crop to sidedress time in the most challenging low-nitrogen availability situations, such as in corn following corn. We find that the best time to apply the rest of the nitrogen is by the V8 development stage at the latest. Some have suggested that sidedress can be delayed until tasseling since corn has only taken up 50% of the total nitrogen by this point. Our research under Minnesota’s conditions does not support this practice. We have also observed that applications delayed until mid-July run the risk of low yields because of inadequate precipitation to incorporate fertilizer into the root zone.
In irrigated coarse-textured soils, planting first and applying nitrogen later is especially important. Because these soils drain quickly, the potential for nitrogen loss through leaching from pre-plant applications is high. Rapid drainage also means that there should be plenty of opportunity to get in the field in a timely manner to apply sidedress nitrogen. In coarse-textured soils (that have low organic matter and low nitrogen-supplying capacity), we consistently find that applying small amounts of nitrogen near planting is adequate for crop needs until a bulk application is applied around mid-June.