Cover crops can help retain the nitrogen from manure applied in the summer or early fall.Field trials with specific livestock manure types to determine how much nitrogen would be available to crops the following year has resulted in Table 9-12, Estimate of Available Nitrogen From Late Summer- and Fall-Applied Manure. To estimate the amount of nitrogen available to the crop, multiply the amount of manure nitrogen applied to the field by the availability factor appropriate for the manure type and application timing. For example, if 45,000 L/ha (4,000 gal/acre) of liquid hog finishing manure is applied in late summer, supplying 285 kg/ha (256 lb/acre) of total N, reduce the N fertilizer application to the following crop by 57 kg/ha (51 lb/acre) (285 lb/acre total N x 0.2 availability factor).Table 9-12 accounts for the volatilization of ammonium-N into the air, the mineralization of organic-N and the loss of nitrate through denitrification or leaching. A large part of the ammonium-N will be lost to the air if manure is left on the soil surface so the proportion of nitrogen available to the crop is greater with incorporated manure. Assumptions have been made concerning the proportion of nitrogen in the ammonium and organic forms (see Table 9-9) and the availability of the organic nitrogen. Urea has been included as an example of how fertilizer nitrogen availability compares with that of manure. These factors can
be used with the results of a manure analysis or the values in Table 9-13, Typical Manure Analysis by Livestock Type, to
estimate nitrogen availability.
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