Because of this and the other sources of N listed above, crop removal alone is not a good estimate of the amount of N to apply. The nitrogen application rate needed to reach maximum yield in South Dakota has been determined through extensive field calibration studies over the past 30 years by SDSU soil fertility researchers. Crops obtain this nitrogen primarily by extracting nitrate from soil, which is in soil at the beginning of the growing season or is made available from any of the nitrogen sources during the growing season. Research has shown that residual nitrate in the top 2 feet of soil prior to fertilization is used as efficiently by crops as fertilizer N or other N sources made available during the growing season. Therefore, when nitrogen fertilizer recommendations are made, the amount of nitrogen determined by the 2-foot nitrate soil test is subtracted from the total nitrogen requirement.
Residual nitrate can be a large source of nitrogen for crops.
The average nitrate soil test from samples submitted to the SDSU soil testing lab over the last 17 years is 55 lb per acre. Therefore, nitrogen recommendations generally go up after wet years and are reduced dramatically, sometimes to zero, after very dry years. Legume credits must also be subtracted from the calculated nitrogen requirement since the nitrate soil test does not measure organic N. Suggested legume credits for crops following the legume are listed in Table 3. For the above example, the 62 pounds of nitrogen needed could be supplied as either fertilizer or manure. If manure was used, an analysis of its nitrogen content would be necessary to determine the proper rate.