Sidedressing Nitrogen on Field and Sweet Corn

Larry Bundy and Carrie Laboski
Extension Soil Scientists

Sidedress nitrogen (N) applications on field or sweet corn can improve the effectiveness of fertilizer N by lowering losses that might occur from preplant-applied N through leaching or denitrification.  On sandy soils, leaching is the greatest risk of N loss, and sidedress or split sidedress applications can help control these losses.  The reason sidedress applications are effective in controlling losses is related to the timing of nitrogen uptake by corn.  Corn uses very little N during the first 4 weeks after planting but then uses most of the N it will need during the next 4 to 6 weeks. This means that sidedress N applied 4 to 6 weeks after planting is available to meet the high crop demand for N during this period and avoids any losses that might have occurred during the first 4 weeks of the growing season.  On coarse textured soils, multiple applications of N often maximize fertilizer N performance.  In this strategy, a small amount of N (10-15 lb/acre) might be applied at planting followed by half of the remaining N requirement as a sidedress application 4 weeks after planting and the remaining half of the N requirement added at 6 to 8 weeks after planting. Alternatively, multiple N applications applied with irrigation water during the 4 to 8 weeks after planting would also be a good N application strategy.  The table below shows the performance of sidedress N applied in split applications at 4 and 8 weeks after planting on sweet corn yields at Hancock.  In this 3-yr experiment, 7 lb N/acre was applied as starter at planting and 170 lb N/acre was split into two equal sidedress treatments at 4 and 8 weeks after planting.

Sweet corn yields with N fertilizer sidedressed at 4 and 8 weeks after planting, Hancock, WI

In general sidedressing N for field or sweet corn on medium and fine-textured soils such as silt loams or silty clay loams, does not improve the performance of the fertilizer N compared to prelant N applications.  The reason for this is that N losses from preplant applied N on these soils is usually low, so the sidedress application does not increase yields by reducing losses.  Sidedress N applications on these soils usually perform about the same as preplant applied N.  There can be some weather-related issues with relying on sidedress applications on fine-textured soils.  The main concern is that soils will remain wet throughout the sidedress application period and adequate amounts of N will not be applied.  In this situation, sidedress N could avoid early-season denitrfication assuming you can get in the field.

Sidedress N applications on fine-textured soils are also an integral component of using the presidedress soil nitrate test (PSNT) to verify manure N credits or other organic N contributions.   In this case, the sidedress N application rate can be adjusted based on the amount of plant-available nitrate-N already present in the soil.  A final issue with sidedressing N on fine- or medium-textured soils is the effect on field work load.  Some farms have other critical field operations to perform at during the optimum period for sidedress N applications.

Summary

On sandy soils, sidedressing N in critical to avoid N losses and optimize benefits from applied N. On medium- and fine-textured soils, it's hard to say that sidedressing will always be superior because N losses from preplant applications are usually small.  Sidedress N applications on these soils involve the risk of weather delays and may conflict with other field operations. However, if growers have successful experience with sidedress N on medium-textured soils, N fertilizer performance will likely be at least as good as with preplant treatments.