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Fluid Journal : Fall 2016
11 The Fluid Journal Fall 2016 Nitrogen Management For Soybeans a Must Economics and environment are two essentials. The Fluid Journal • Official Journal of the Fluid Fertilizer Foundation • Fall 2016 • Vol. 23, No. 4, Issue #94 Dr. A. L. Shober and R. Taylor ▼ DOWNLOAD Summary: Summary: Growers are unlikely to see yield increases when applying supplemental fertilizer or manure N to soybeans, except in the case of high- yielding irrigated soybeans. Growers should consider applications of supplemental N only when yield of irrigated soybeans consistently exceeds 60 bu/A. Soybeans are the second most widely produced crop in Delaware, ranking just behind corn for grain in 2012. Approximately 168,000 acres of soybeans were produced in Delaware with an average yield of 42.5 bu/A. Producers seeding to improve yields of soybeans and overall profitability may be considering application of supplemental N fertilizer or manures. However, growers should consider both economics and the environment when deciding to apply supplemental N to soybeans. The purpose of this article is to help guide decisions about application of supplemental N to soybeans. Nitrogen sources The N needs of soybeans are quite high due to the higher protein content (= 40%) in soybean grain. Soybean N removal in grain is estimated at 138 lbs/A for soybeans yielding 40 bu/A, respectively. This is roughly equivalent to the amount of N removed by 200 bu/A grain corn. An irrigated soybean crop yielding 70 bu/A would remove about 242 lbs N/A in the grain. The main sources of N that are available to meet the N needs of soybeans are the atmosphere and the soil. In some cases, commercial fertilizers and/or manure may also be used to meet N needs of soybeans. Soybeans are a legume and are able to obtain N from the atmosphere because they form a symbiotic relationship with N- fixing bacteria called Bradyrhizobium japonicum. These N-fixing bacteria colonize the roots of the soybean plant forming nodules. Within these nodules the bacteria are able to convert (or fix) N2 gas from the atmosphere to ammonium (NH4+), which is a plant available form. The relationship is considered to be symbiotic because the soybean plant provides a food source (carbon) for bacteria and the bacteria provide N to the soybean plant. Maximum N2 fixation potential by the soybean is estimated to be 300 lbs/A under ideal environmental conditions (e.g., adequate soil, soil moisture, fertility, and sunlight; no compaction in root zone, etc.). Inorganic N Soybeans can also obtain inorganic N from the soil in the plant-available forms of ammonium (NH4+) or nitrate (NO3-). Some plant available N may be residual in the soil, meaning it was left over from fertilization of previous crops or a breakdown of crop residues and residual manure applications. Soil organic matter is also a source of plant-available N. When organic matter is broken down by soil microbes, the organic N is converted to NH4+ via a process called mineralization. Maximum soil N mineralization is estimated at 100 lbs/A, with less mineralization expected in lower organic matter soils. Supplemental N Historically, application of commercial fertilizers and/or manures to soybeans was not recommended because N2 fixation and soil N was considered to be adequate to meet the N needs of soybean crops. However, due to improved genetics, expansion of irrigation, better weed control, and other production factors, soybean yields in many areas are increasing. There is some evidence that high-yielding soybeans (>60 to 80 bu/A) may benefit from supplemental N applications because N2 fixation and soil N may not be adequate to meet crop needs at high yields (Figure 1). Potential consequences There is no cut and dry recommendation about whether or not to apply N fertilizers or manure to a soybean crop. However, there are situations when application of supplemental N to soybeans is NOT recommended because it can limit yield, waste money, or have a negative impact on the environment: • Early-season application of manure and/or commercial N because of potential delayed and reduced nodulation and reduced activity of the nodule bacteria. • When supplemental N is applied to soybeans, the plants essentially “get lazy” because it is easier to take up the supplemental N than it is to establish a symbiotic relationship with soil microbes. For example, nodulation of soybeans planted at the Carvel Research and Education Center in Georgetown DE in 2014 was greatly reduced when fertilizer at planting with N (as urea fertilizer) at 100 lbs/A, compared to plants receiving no supplemental N (Figure 2). • Nitrogen fixation by a soybean decreases exponentially as N