A soybean crop can contain a large quantity of nitrogen (N) in the form of proteins, which are digestible by all animals. (It is important to note that soybean meal must be heat treated when fed to monogastrics, like poultry and swine.) Soybeans have high crude protein content, which results in roughly 5 to 7 percent N content in grain on a dry weight basis. Based on UD variety trial yields, we estimate current yield potential for soybean grown under ideal conditions to range from 65-80 bu/A for full season beans and 40-50 bu/A for double crop beans. As such, the N removed in the grain can range from 120-210 lb N/A for double crop beans and 195-340 lbs N/acre for full season beans. Even with these high removal rates, soybean crops are seldom purposely fertilized with N. Why?
Soybean is an efficient legume, which means that the plant does a good job of converting atmospheric nitrogen gas (N2) into plant available N compounds (mainly ammonium) through a process called N fixation. Soybeans form a symbiotic relationship with Bradyrhizobia bacteria, which colonize roots of the soybean plants. The plant must supply the energy needed by the symbiotic bacteria as fixed carbon (sugars, ATP, etc.), but in return the plant benefits from conversion of atmospheric N to plant available forms. Many people automatically assume that legumes fulfill their entire N requirement by N fixation and do not remove N from the soil N pool; however, this is incorrect. Soybean N fixation, as reported in the literature, ranges from 40-75%. Even an excellent N fixing soybean variety will only obtain 60-75% of the plant’s N requirement from N fixation.
If the crop only fixes a portion of the N required for grain production, from where does the remainder come? The obvious answer is that the N comes from the soil, which occurs through mineralization of soil organic matter and other residual sources of N. So, why do we not generally fertilize a soybean crop with N? Legumes and the N-fixing bacteria have several special characteristics that limit the effectiveness of supplemental N fertilizer. The first special characteristic is that the plant has a tap root and not a fibrous root system. This means that if grasses are growing in competition with tap rooted soybean crop, the crop with the tap root is at a distinct disadvantage when competing for the soil available N. When compared to the fibrous root systems of grasses, the tap root system of soybean has access to a significantly smaller volume of soil from which it can obtain N.
The second special characteristic is that the symbiotic relationship between the plant and the Bradyrhizobia is adversely affected by soil available N levels. If the bacteria are already established in root nodules, N fixation decreases as soil N levels increase. If soil N levels are high at the time the plant is emerging or during the early establishment phase, fewer, smaller, and less active nodules will form on the soybean roots. If N is applied as commercial N or manure to a soybean crop and the crop is able to take up adequate N from the soil, the symbiotic relationship with the N fixing bacteria may account for very little of the N used during growth and development. If the soil N level remains adequate until just before flowering or sometime during flowering, the crop can come under yield limiting N stress at a critical time during development. It can take two or more weeks for nodulation to occur and active N-fixation to begin to supply the N needs of the crop. Under these circumstances, N fixation occurs too late and yields are significantly reduced. For these reasons, University of Delaware does NOT recommend pre-plant or early season application of N to soybean, especially in the form of manure or compost. Use a good inoculant instead.
In a recent article, we discussed heavy residue impacts on the need for N fertilizer. For a soybean crop, heavy residue (if it does not interfere with stand establishment) could help immobilize soil N and create a low soil N level, which would be conducive to formation of nodules by applied soybean inoculants.
Are you irrigating your soybean crop? If so, there is a chance you are already supplementing your soybean crop with N. Due to elevated concentrations of N in groundwater in some areas (particularly in parts of Kent and Sussex Counties that irrigate from the surficial aquifer), soybean (and other crops) may receive small doses of N during irrigation events. Similarly, irrigation of soybean with wastewaters will provide some additional N to the soybean crop. If concentrations of N in the irrigation water are consistently high (as might occur with wastewater irrigation), nodulation of soybean will be suppressed and the crop will obtain N from the irrigation water.
Is there ever a situation where N fertilization of soybean is beneficial? There is some evidence that the crop benefits from in-season application of N when yields are high (≥ 70 bu/A). In high yielding conditions, the amount of N available from fixation and the soil may not be adequate to support maximum yield. Under these situations, researchers have suggested that an application of N at pod fill (approximately R5) can increase soybean grain yields. However, this is ONLY feasible if the crop is irrigated and can be fertigated. Otherwise, equipment damage to the crop that would occur during pod fill N application would cause too much damage and result in a yield loss to growers.
Sulfur deficiency is another fertility issue that we have yet to observe, but that could become more important in the future. The amount of sulfur (S) supplied by deposition from the atmosphere has declined significantly over the last decade due to air quality regulations. Since S is a component of some critical amino acids required for protein synthesis, the heavy uptake or fixation of N by soybean requires a corresponding higher uptake of S. Although the soybean tap root may be able to reach the S stored in the subsoil of many of Delaware’s sandy soils, compaction issues and acid subsoil issues could potentially limit soybean yield potential. Since S leaches so readily in sandy soils, the increased use of ammonium sulfate in corn production systems may not address the need for S by the soybean crop. Fertilization with gypsum, Epsom salts, or K-Po-Mag can supply the needed S.
Another fertility issue that growers often encounter in soybean crops is manganese (Mn) deficiency. There is still debate about the impact of Roundup Ready™ soybeans on the ability of the crop to take up Mn and metabolize the micronutrient. Regardless, low native soil Mn levels and high soil pH can also lead to Mn deficiency symptoms. Manganese deficiency symptoms appear first on the newest leaves, showing up as interveinal chlorosis that can become so severe that the newly emerged leaves appeal almost white instead of green. Foliar Mn treatments are very effective at maximizing yield potential; although, the recommended rate of Mn application depends on the growth stage of the crop and available leaf area. Leaf area is important to allow the crop to take up enough Mn from the foliar treatment to satisfy the Mn need. Chelated Mn, as well as Tecmangam (manganese sulfate), work very well to correct Mn deficiency. We recommend 0.5 to 2.0 lbs Mn/A, usually applied as one application provided the leaf area is adequate.