Soybean has high protein content, which is rich in N, so its needs for N are high.
Because soybean can fix its own N, farmers have traditionally taken a "hands-off" approach to N fertilization. The exception is in fields not recently planted to soybean, in which case a seed-applied "inoculant" of rhizobia bacteria has been used. This has been the extent to which most growers have worried about N fertilization for soybean, and it has largely been a successful approach for over half a century.
Today, however, some soil fertility recommendations for soybean are including N fertilizer applications, often in the range of 20 to 40 lbs/acre but sometimes higher. At the same time, other recommendations are consistent with conventional wisdom - inoculating new soybean fields with rhizobia and nothing more. These varying recommendations may have growers wondering what has changed. The answer is that soybean fertility guidelines are evolving to account for increasing N demand from higher-yielding soybean. This article will discuss the N needs of today's higher-yielding soybean crops, sources of N supply to the crop, and whether N fertilizer applications may be needed for maximum soybean yields.
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.
To summarize, soybean requires a large amount of N. Because only a portion of this can be supplied by N fixation and soil mineralization, growing higher-yielding soybean will likely require another source of N.
At about 60 days after planting, or about the R4 growth stage, soybean begins to move N from the vegetative parts of the plant to the grain. This might suggest that the best time to apply additional N is prior to R4 (during the early reproductive growth stages) so that fertilizer N is readily available to the plant by R4. If this applied N could delay or minimize the shift of N from the vegetative parts to the seed, it may prolong the duration when the plant remains green and is moving carbohydrates to the seed, and therefore may increase overall grain yield.
Although an N fertilizer application during early reproductive growth stages is during a period of great demand by soybean, it is not known if the N applied would be additive to the N fixed by the plant. Conversely, it could decrease N fixation by some amount, even up to the total quantity of N fertilizer applied, thus resulting in a zero net gain in available N to soybean. Unfortunately, as stated previously, there is not a straightforward answer at this time.
Supporting research: A slow-release N study
How might N fertilizer be applied to soybean without adversely affecting N fixation? An approach taken in a Nebraska study was to apply 160 lb N/acre of slow-release N fertilizer (polymer coated) before planting and place it 8 inches below the soil surface midway between the rows (Salvagiotti et al., 2009). The placement at 8 inches depth was intended to avoid or minimize the reduction of N fixation, since this put the N fertilizer below the zone where most N fixation occurs. Using a slow-release form of N allowed the application to be made prior to planting but delivered the N fertilizer closer to the plant's peak demand (during reproductive growth stages). In this Nebraska study, the deep placement of slow-released N fertilizer was compared to broadcast applications of ammonium nitrate either split before planting and at the V6 growth stage or all applied at the R5 growth stage. The check treatment had no N fertilizer applied.
Field with soybean planted for the first time. Dark strips were inoculated with rhizobia bacteria; light strips were not.
The results showed that grain yields from all 3 N treatments were the same, achieving 81 bu/acre, which was 3.6 bu/acre greater than the untreated check. This relatively small yield response to N did not justify applying 160 lb N/acre. It is unknown whether less N fertilizer would have been sufficient to increase yield.
Results also showed that the deep placement of slow-release N fertilizer was successful in not reducing the amount of N fixed by the soybean, as the 180 lb N/acre fixed by the plant for this treatment was the same as that fixed by the check treatment. This compared to 140 lb N/acre fixed for the broadcast ammonium nitrate treatments. However, the N fixed in this study was considerably less than average for 80 bu/acre soybean (220 lb N/acre) when compared to the many studies represented in Figure 1. Additionally, N uptake attributed to soil was between 160 and 200 lb N/acre - much more than typically attributed to soil mineralization (80 to 100 lb N/acre). For this Nebraska study, fixed N plus soil N totaled 340 to 380 lb N/acre, which is usually sufficient to produce 80 bu/acre soybean (Figure 1). Thus, the lack of response to the N fertilizer could have been a consequence of a soil that was already providing plenty of N by mineralization.
* John P. Schmidt, Pioneer Research Scientist, Soybean Production, Champaign, Illinois.
The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation.