The Nebraska Extension Circular, Nutrient Management Suggestions for Corn (EC117), has been updated. The major changes are in accounting for residual soil nitrate-N when determining fertilizer-N rates, and an alternative for determining fertilizer phosphorus rates.
EC117 is free and can be downloaded at extensionpublications.unl.edu/assets/pdf/ec117.pdf.
Residual soil nitrate-N can be estimated by taking a single 0- to 2-foot soil sample. A default value for the 2-foot depth of 3.0 parts per million is suggested for medium or fine texture soil, and 1.5 ppm for sandy soils. The earlier publication recommended sampling to the 4-foot depth with four 1-foot samples.
This revision was justified because of the overall improvement in fertilizer-N use efficiency over the decades and the recent interpretation of results from thousands of soil samples.
There still are cases of excessive residual nitrate-N when sampling to the 4-foot depth is justified. For example, if soil samples indicate more than than 15 ppm nitrate-N in the 2-foot depth, or if there was little harvested N with the previous crop such as due to hail damage, then 4-foot samples should be taken.
Based on results from research conducted at the Eastern Nebraska Research and Extension Center near Mead, Haskell Agricultural Lab near Concord, and West Central Research and Extension Center at North Platte over six years, an alternative means to determining the fertilizer rate for phosphorus was developed.
With this alternative, the fertilizer-P rate is equal to harvest P removal if Bray-1 P equals less than 20 ppm for corn following corn or less than 15 ppm for corn following soybeans. Further explanation and an example calculation are provided in EC117. The publication also links to a downloadable fertilizer-P rate calculator.