Source: University of Minnesota Extension
By Anne Struffert, Extension Educator and Fabián Fernández, Nutrient Management Specialist
In the Central Sands of Minnesota, planting is under way. With a week of warm and windy weather, things are drying out and warming up quickly. While most growers have made decisions on variety and tillage, one thing you may not have nailed down is when and how many times should you fertigate nitrogen on corn.
Loss potential in the spring is almost always high on sandy soils. With a combination of snow melt, excess rain, and a crop that is not yet needing much water or nitrogen, much of the nitrogen that we apply can leach out of the profile because it has nothing to hang on to. Because of this we suggest delaying any nitrogen application until the V2 development stage. If you need a little peace of mind and want to apply 10 to 20 pounds at planting, that is fine, just remember to keep that rate low.
After the first application, you can start to spoon feed that crop with fertigation. Research shows that doing between 3 and 4 splits starting at V2 and finishing by V12 development stages (approximately the middle - end of June) works well. One thing to keep in mind though, is that fertigation might not be the best option every year for the early applications because the crop might not need irrigation water. That said, whatever way you decide to apply your nitrogen, the goal is to apply just enough to feed the crop until about the middle of June and then apply the rest of the N needed for the crop. This is important for a few reasons:
First, after that time the loss potential is usually low because there is no excess moisture. The crop is also taking up nitrate quickly, so most of what you apply should stick around.
Second, in trials we see little to no advantage to keep applying after V12. We’ve seen similar yields fertigating between V12 to tassel vs. the 3-4 split approach, so there’s no need to waste time after V12.
Third, you don’t want to be in a situation where you haven’t applied enough nitrogen for when the crop needs it. When fertigating, we are giving that crop a little nitrogen at a time, and during late June into July that crop is putting on a lot of biomass so that crop will need more nitrogen (fig. 1).
With those points in mind, applying after V12 is still acceptable, but we do not suggest applying after tasseling. We see no benefit and sometimes a decrease in yield when nitrogen is delayed that far.
Some other considerations to keep in mind when irrigating is the amount of water we are putting on. There are several ways to track how much water your soil needs to keep that crop happy. Tensiometers, irrigation scheduling, or even just going out to the field and doing a hand moisture test are all good options. Learn more about those at https://z.umn.edu/fertigation. We know nitrate moves with water, and if your soil is already at field capacity and you irrigated, you’re increasing the risk of nitrogen loss. Also, for fertigation to work well you must ensure your system is watering uniformly.
Another question is how much total nitrogen to apply. Through research we calculated that 209 lb/N acre optimizes yield for continuous corn for a nitrogen price/corn value ratio of 0.10. In corn after soybean the rate should be reduced by 30 lb/N acre. More details are available here.
You can also apply this information to sites with irrigation where fertigation is not an option. Remember that without fertigation, each trip across the field implies additional cost. For that reason, the best thing to do in those situations is to try to apply a limited amount of nitrogen around the V2 stage, just enough to feed the crop until you do the next application. Then apply the rest of the N as late as possible, but no later than V12, taking into account crop height that might prevent driving in the field, depending on the equipment available.