As Figure 1 indicates, average N fixed by soybean increases linearly with increasing yield, but only a portion of the total N requirement is met through N fixation (about 50% to 60% of total N requirements at yields of 50 bu/acre or less). Based on the average of the 100+ studies represented in Figure 1, at a yield level of 60 bu/acre, fixed N provides about 180 lb of the 270 lb N uptake in soybean, or 65% to 70% of the total required N. For yields up to 60 bu/acre, the difference between total N uptake (i.e., plant requirement) and fixed N is usually provided by soil sources.
The N budget in Figure 1 also illustrates that there may be a small N deficit for yields between 60 and 80 bu/acre, which means that yield could be restricted because of too little N. Realistically, conditions that are favorable for top soybean yields are usually conducive to high soil mineralization as well, so N would not always be limiting in the 60 to 80 bu/acre range. However, as soybean yields continue to increase and yields in this range and higher become more common, N fixation and soil N mineralization will reach capacity in many growing environments. Thus, an increasing number of N shortfalls are almost certain to occur based on the current understanding of this system, particularly at yields near 100 bu/acre. As the graph shows, soybean's upper limit for N fixation (considered to be about 300 lb/acre) combined with the upper limit of the soil supply (usually less than 100 lb/acre) are insufficient to meet the needs of a 100 bu/acre soybean crop (Salvagiotti et al., 2008).
Understanding soybean N needs by comparing to corn
Another approach to understanding soybean N needs is to compare it to corn, a crop for which growers routinely estimate N needs. Consider a situation of growing corn without application of N fertilizer: if soils can provide 80 lb N by mineralization, resulting corn yields may approach 100 bu/acre, which contains 80 lb N in the grain. To achieve corn yields above 100 bu/acre would require supplemental N beyond what soils are able to apply, i.e., application of N fertilizer. This same 80 lb N supplied by the soil, when added to N fixed by the soybean crop, is sufficient to produce 50 to 60 bu/acre of soybean (Figure 1); achieving higher yields may require supplemental N.
N Credit for corn following soybean?
The N budget also makes an important point about the prospect of an "N credit" from a soybean crop that may reduce the N fertilizer requirement for a subsequent corn crop. Because soybean does not fix enough N to satisfy its own needs, but rather, removes N from the soil at both low and high soybean yield levels, the concept of an N credit is not supported. Nevertheless, the practice of applying less N to a corn crop following soybean is still valid, but for a different reason.
Soil microbes that digest crop residue use the carbon (C) remaining in plant materials as an energy source. As microbial populations increase in the presence of carbon-rich residues, they also require N for cellular growth and metabolism. Because most crop residues are high in C but not in N (C:N ratio is high), microbes use N that is available in the soil. This temporarily “ties up” the N, making it unavailable for other uses, including crop uptake. Eventually, however, N is released back to the soil as crop residues are decomposed and microbial populations decline. Soybean crop residue contains considerably more nitrogen than corn residue, which speeds up the process of residue decomposition and ties up less N in the subsequent corn crop. Thus, less N needs to be applied to corn following soybean.