Many people believe that split-applying nitrogen (N) rather than doing a single application near planting is a better approach to N management, but is it really?
Inconsistent weather causes inconsistent crop response
Split applications of N fertilizer are undoubtedly beneficial in areas of Minnesota with sandy soils that have low nutrient and water holding capacity. However, the utility of split applications in other parts of the state has been inconsistent and is largely influenced by weather conditions.
In very general terms, recent studies in fine-textured soils across Minnesota and other Midwestern states show that excessive precipitation after a single pre-plant N application results in greater susceptibility for early season N loss. On the other hand, a lack of precipitation after in-season N application reduces grain yield in comparison to a single pre-plant application. This is because N fertilizer is positionally unavailable for crop uptake.
Unfortunately, the accuracy of season-long weather forecasting is still low and limits our ability to preemptively adjust management decisions to consistently maximize input efficiency in response to weather conditions. This means that management decisions often have to be made based on the results of research trials from past years and previous experience on the likelihood of a desired outcome.
When split-applying, how much N should be applied pre-plant?
Corn N demand is low until the V6 stage, so not a lot of N fertilizer needs to be applied pre-plant. A large study across several Midwest states, including Minnesota, where sidedress applications were done around V8-V9 showed that around 95% of the time there was no difference in crop yield between a pre-plant application of 40 or 80 lbs N/acre. Most of these sites were corn after soybean. When corn follows corn, applying 80 lbs N/acre pre-plant is probably a better alternative.
When is the best time to do a sidedress application?
A recent 15-site-year study in Minnesota indicated that split applications in coarse-textured soils between V4 and V12 increase corn grain yield by 50 to 90% compared to a single pre-plant application regardless of weather conditions.
On fine-textured soils, however, pre-plant applications were as good or better than split applications and applying after the V8 stage sometimes actually reduced yield. For this reason, if you want to sidedress N in fine-textured soils, make sure you do it before V8.
When split applying, is it possible to reduce the overall N rate?
A large study conducted across 49 site-years in the Midwest, including several sites in Minnesota, showed that the economic optimal N rates (EONR) was overall 6% (9 lbs/acre) lower with split application compared to a single preplant application.
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However, most of the difference can be attributed to sites with soils with high N loss potential. In the majority of cases, there was no difference in the amount of N required to optimize yield between split application and a single preplant application.
Another study conducted over the last five years on a corn-soybean rotation in a poorly drained soil in southcentral Minnesota showed similar results. At this site, we compared split applications in fields with tile drainage and fields without tile drainage conditions.
In undrained soils, where the potential for N loss from denitrification is greater in wet springs, split application required 15 lbs/acre less N fertilizer than the single pre-plant application to achieve the same yield.
However, in drained soils, the split application required 16 lbs N/acre more than the single pre-plant application to achieve the same yield. So, while soil and weather conditions clearly influence N response, there is not enough evidence to support a reduced N rate when split-applying in most fine-textured soils.