Land application of animal manures can supply N needed for com production. The amounts ofN supplied, however, vary greatly with source of manure, type of storage, method of application, time of application, rate of application, and weather after application. A major problem is that substantial losses of plant-available N by ammonia volatilization, leaching, denitrification, and/or immobilization frequently occur soon after the manure is applied. Another problem is that it is difficult to

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... ne how rapidly N in the manure becomes available to plants. These problems make it difficult to estimate how much fertilizer N should be applied. Soil testing for nitrate in the surface foot of soil when plants are 6 to 12 inches tall is gaining acceptance as a tool for assessing fertilizer N needs. The soil test reduces uncertainty in fertilizer N needs because it enables site-specific assessments that reflect any losses or gains in plant-available N that occur before the soils are sampled (Blackmer et al., 1993) . The assessments are made early enough that more N can be applied if needed.