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Cover Crop Nitrogen Use on Fallow Land
Nitrogen (N) available for crop growth is subject to loss several ways. Excessive moisture may cause leaching or denitrification when N is lost as a gas. Cover crop or weeds will use N for growth, reducing available N until residue breaks down.
Fallow fields are especially subject to these losses. In past years fallow field soil tests have shown nitrogen levels as low as 30 lbs/acre. In 1991 soil test lab numbers for NE ND showed 40% of fallow fields testing less than 80 lbs of N. Seventy-five percent were less than 100 lbs of N/acre. Areas that received much above-normal rainfall, sandy soils and fallow fields with abundant cover crop growth should be soil tested for N. Nitrogen loss is only one reason for low N levels following fallow. Another reason is that some soils have little readily mineralizable N left after mining the soil for 100 years. However, N can be lost through leaching and be tied up by cover crops so that the soil test does not reveal its presence.
In fallow fields with cover crop, N levels may be lower than expected. Boot stage small grain will use about 60 lbs of N and mustard with pods forming will use about 80 lbs of N. Less crop growth or thinner stands will use less nitrogen. Only 1/2 - 2/3 of this N will become available after spring incorporation but not before early to mid- June. This delay in nitrogen release may result in N deficiencies early in the growing season.
Depending on soil N available, cover crop amount and growth stage and yield goal, N may be needed. Nitrogen in cover crop complicates management decisions on how much N may be needed, if any. For example, a fallow field with a uniform cover crop of boot stage barley tests 40 lbs N/acre. Fifty bushels of wheat will need about 125 lbs of N for adequate growth. 125 lbs of N less 40 lbs N supplied by the soil equals 90 lbs of N to be supplied by other sources. About 30-40 lbs of N will be supplied (1/2-2/3 of 60 lbs in the crop) by the decomposing barley next spring and summer. This leaves 50-60 lbs to be applied this fall or spring from N sources. Less cover crop growth will use less N and also have less to contribute to next years crop. Use care when estimating N use and contributions from cover crops, because the rate of mineralization is not predictable.
Test fallow fields for N. Evaluate N needs carefully after receiving test results. Fallow fields have high yield capabilities. Make sure fertility levels meet those capabilities.
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