Estimated Nitrogen Release or ENR is a calculated estimate of how much nitrogen will be released through the growing season from the organic matter (O.M.) or humus-like materials. Therefore, as the % O.M. increases, so will the ENR.
This calculation is based on the “rule of thumb” that organic matter contains 5% nitrogen. The rate at which organic matter will decompose and release nitrogen depends on many factors, but those of greatest effect are soil type, moisture and temperature. Because of this we adjust the ENR for geographical region (zones are listed on Midwest report), soil type (see Table 1 in the Midwest Laboratories, Inc. Agronomy Handbook), and crop (warm season vs. cool season).
Midwest uses ENR in computing nitrogen recommendations so that we do not over apply nor under apply nitrogen to our crops. Questions sometimes arise as to how to explain the use of the ENR to the farmer. Following is an example of how one may explain its use regarding a corn recommendation:
A 150-bushel corn crop will remove 150 lbs. per acre in the grain and 75 lbs. nitrogen per acre in the stover meaning the total crop need is 225 lbs. nitrogen per acre. If we assume we have an organic matter level which will result in an ENR of 65 lbs. nitrogen per acre, we then have to supply the crop an additional 160 lbs. per acre through fertilizer or other plant foods. Realize that the Midwest nitrogen recommendation has already been adjusted using the ENR.
Let’s further assume that we have a nitrate level in the surface soil sample of 30 lbs. nitrogen per acre. Our nitrogen requirements would not be 130 lbs. Remember that nitrogen applications of fertilizer are not 100% efficient. Nitrogen is subject to losses by volatilization, immobilization, denitrification, and leaching. We can normally assume an efficiency factor of 60% to 70% with a preplant application. With an assumption of 70% efficiency, we would have a recommendation of 185 lbs. for 150-bushel corn.
150 bu./A X 1.5 = 225 lbs. nitrogen needed
-65 lbs. ENR
-30 lbs. NO3-N
x 70 % N efficiency factor
185 lbs. nitrogen recommendation
As you split your applications of nitrogen, you reduce your risk of loss and increase your efficiency. If you increase your efficiency factor to 90%, you would only need 145 lbs. nitrogen. This is where adjustments can be made in nitrogen recommendations which are received from the laboratory.
Another adjustment in making nitrogen recommendations would be credit for the previous crop (legume). Remember that this nitrogen is in the crop residue and roots. These must be decomposed before the nitrogen is available. If you are planting a cool season crop following a legume, no adjustment should be made as time has not permitted crop residue to be decomposed. For corn following soybeans we suggest a 20-lb. N credit and 50 lbs. for a good stand of alfalfa.