Iowa State University's new nitrogen fertilizer recommendations for corn (outlined in Pm-1714) do not involve calculating credits for N supplied by previous soybean crops. However, corn-following-soybean is considered in a different crop category than corn-following-corn. This change was made because research indicates that the primary mechanism responsible for differences in N fertilizer requirements does not involve carryover of extra N fixed by the soybean crop. Instead, the primary mechanism relates to the amounts of residues returned to soils. The following 10 points outline what research has shown about the factors responsible for the difference in N fertilizer requirements.

1. Nitrogen response trials in Iowa have shown that "economic optimum" rates of N fertilization usually are lower for corn-following-soybean than for corn-following-corn. Such observations suggest that there is some type of carryover of N from soybean to corn.
2. Soybean crops "fix" N from the atmosphere to acquire N needed for growth and seed production. Some of the fixed N is left in the soil and, undoubtedly, is used by the next crop.
3. Estimates of the amounts of N fixed by soybean crops indicate that substantially less N is fixed than is removed from the field during grain harvest. In contrast, production of corn grain removes less N from fields than usually is added as commercial fertilizer. Such information suggests that N fixation by soybean (summarized in #2) is irrelevant when trying to explain the normal difference in the fertilizer requirements between corn-following-corn and corn-following-soybean crops (summarized in #1).
4. Paired response trials at many sites during the past decade have shown extreme variability in the difference in N fertilizer requirements between corn-following-soybean and corn-following-corn. This variability is not consistent with the old idea that the difference in fertilizer requirements is caused by carryover of extra N fixed by the soybean crop. Moreover, the observed variability is great enough that adjusting for the average difference in fertilizer requirement will result in an incorrect adjustment in most individual fields. It should be noted that recommendations based on N credits were developed to encourage producers to adjust fertilization rates for the average amount of N that seemed to be carried over.
5. It is well known that microorganisms that decompose corn residues in soils often compete with crops for plant-available N. This competition occurs as the microorganisms immobilize (i.e., convert to organic forms that are not available to plants) nitrate and exchangeable ammonium. The competition occurs whether the plant-available N is derived from fertilizer or soil organic matter. Increases in the amount of residue returned to the soil increase the amount of N immobilized and, therefore, decrease the amount of N immediately available to plants.
6. Microorganisms that decompose soybean residues also immobilize N and compete with the next crop for N. However, soybean leaves less residue than corn, and this difference results in more plant-available N for corn that follows soybean. Estimates of the average difference between crops in the amounts of N immobilized are generally similar to the amount that has been recommended as N credits. Within any given year, therefore, differences in the amount of N immobilized during residue decomposition provide a reasonable explanation for observed differences in the N fertilizer requirement.
7. Pound for pound, soybean residues decompose faster than corn residues due to differences in the nature of the residues. In addition, soybean crops tend to be harvested earlier than corn crops. Immobilization of N during soybean decomposition, therefore, is likely to be completed sooner than is immobilization of N during corn residue decomposition. Earlier immobilization, especially that occurring in the fall, reduces the potential for N losses by leaching and denitrification between cropping seasons. Year-to-year variability in the amounts of residue, times of decomposition, and amounts of N lost by leaching and denitrification between seasons could explain the observed variability in the difference in fertilizer requirements. Recognizing the factors that cause this variability should help efforts to predict N fertilizer needs at specific sites in specific years.
8. Long-term studies have shown that concentrations of soil organic matter decrease more rapidly with corn-soybean cropping sequences than with continuous corn production. Such observations indicate that the long-term difference in fertilizer requirements are best explained by the tendency of soybean to promote a greater conversion of soil organic-N to plant-available N. Much of this effect should be attributed to slower formation of new organic matter because soybean returns less residue to the soil than does corn. Soybean crops also may cause a more rapid mineralization of old soil organic matter (i.e., any organic materials present before soybean was grown).
9. Explanations of the apparent carryover of N based on the decomposition of residue (see #6) and the loss of soil organic matter (#8) describe different aspects of the same overall process. That is, N immobilization and new organic matter formation are linked during the process of corn and soybean residue decomposition in soils. Also the net amounts of immobilization observed during residue decomposition under field conditions depends on the amount of old organic matter mineralized. The explanation associated with the effects of residue decomposition (#6) is more useful when trying to explain or predict the N fertilizer requirement for a given site and year. The explanation associated with loss of soil organic matter (#8) is more useful when trying to explain the origin of the N that seems to be carried over from soybean to corn.
10. The addition of fertilizer N in quantities greater than needed to maximize plant growth should not be expected to increase concentrations of soil organic matter. Indeed, there is evidence that adding extra N may accelerate losses of soil organic matter through a process that has been called a "priming effect."

These 10 points can be summarized by one statement: Corn-following-soybean requires less N fertilizer than does corn-following-corn because soybean crops cause a greater net mineralization of N from soil organic matter. This explanation really is no more complicated than the popular explanation that the major mechanism by which nitrogen seems to be carried over from soybean is the carryover of extra N fixed by soybean crops. The advantage of using the new explanation is that it is consistent with all observations being made, and it provides a better basis for understanding what is really happening in soils.

The exact mechanism by which N seems to be carried over from soybean to corn was relatively unimportant to producers when it was not practical to adjust rates of fertilization for soil and weather conditions. Under such conditions, it was reasonable to have recommendations to adjust for the average difference in fertilizer requirements.

However, an accurate understanding of these concepts is needed by producers today, who have the opportunity to use "precision farming technologies" that enable site-specific applications of N. Such knowledge is needed immediately to design, select, and use technologies that are likely to substantially improve N management. Producers who take the time to learn how to use all these new technologies are well advised to take a little more time and use the late-spring test for soil nitrate, and the end-of-season test for cornstalk nitrate, to learn more about the key factors controlling N availability in their soils.