Drought conditions throughout much of Wisconsin in 2012 resulted in yields that were less than growers had been planning for when they planted in the spring. Under drought conditions, there is the possibility that the drought stressed crop did not use all of the nitrogen that was applied. This unused (or residual or excess) N will remain in the soil profile until it is used by another crop or leached. Situations with the greatest potential for excess N to remain in the soil profile after the 2012 crop include fields with drought stressed corn, where manure was applied for the 2012 crop, or where forage legumes were grown in 2011. If fall, winter, and early spring rainfalls are normal or below normal, it is likely that unused N from 2012 will still be in the soil profile in spring 2013 and be available for the 2013 crop.
To adjust N applications to corn fields in 2013 where residual nitrate is likely, a preplant nitrate test (PPNT) can be taken prior to planting corn in the spring. Soil samples for PPNT are collected at the 0-1’ and 1-2’ depth. In the spring prior to planting, it is typical to see 50 lb N/a in the soil profile. Therefore, 50 lb N/a should be subtracted from the PPNT results to arrive at a N credit. This N credit should then be subtracted from the top end of the corn MRTN rate guidelines. For more information on the PPNT and MRTN consult Chapter 5 in UWEX Publication A2809 Nutrient application guidelines for field, vegetable, and fruit crops in Wisconsin.
In an effort to assess residual soil nitrate following the 2012 corn crop, a soil nitrate monitoring network was developed. Soil samples were collected from 0-1’, 1-2’, and in some cases, 2-3’ deep in fields throughout the state after corn was harvested and after adequate rainfall occurred to allow sampling with depth. The amount of residual nitrate at each location along with some field information can be found at: http://uwlab.soils.wisc.edu/soilnitratemonitoring. The amount of nitrate remaining in the soil profile across the state is highly variable. Coarser textured soils typically had low amounts of residual N. Fields with higher N application rates and/or manure application tended to have higher residual N. The greatest amount of residual N was 325 lb N/a. If this amount of nitrate was found in PPNT samples, the N credit would be 275 lb N/a (325-50=275); more than enough to grow a crop of corn with no additional fertilizer or manure. These results suggest that growers should strongly consider taking soil samples for PPNT in the spring to adjust N applications to improve profitability. Soil samples will also be collected from the soil nitrate monitoring network fields in spring 2013 and results posted as soon as data are available.