Results from a two-year (mid 1990’s) Kansas State University study at four irrigated sites in Kansas show that late-season application of N to soybeans at the R3 growth stage will increase soybean yields. These results, coupled with similar results in other states, suggested that retail input suppliers, public and private labs and consultants who make fertilizer recommendations should consider N applications on irrigated soybeans with high-yield potential. Six of the eight site-years were responsive and showed an average soybean yield increase of 6.9 bu/A or 11.8 percent with late-season N fertilization. Yields at these six sites ranged from a low of 56 bu/A to a high of 83 bu/A. The researcher concluded that producers of high yielding soybeans (greater than 55 bu/A) would benefit from a late-season application of N at the 20-lb/A rate of N.
Fast forward to 2016 where University of Delaware researchers arrived at very similar conclusions. These researchers found that application of supplemental N may provide a yield benefit for high-yield soybeans, but only in cases where expected yields are 60 to 70 bu/A or higher. In summing up their research, they concluded that Growers are unlikely to see yield increases from late-season (R3) supplemental fertilizer N in situations where yields are not likely to exceed 60 bu/A. In production scenarios where yields are likely to be less than 60 bu/A, application of supplemental N is more likely to result in unnecessary expense and increased environmental impact.
Access Kansas State University Fluid Journal Soybean N article
Access University of Delaware Fluid Journal Soybean N article
Depending on the supply of ammonium thiosulfate (ATS, 12-0-0-26S) at times and the purchase price differential between ammonium sulfate and ATS, there are sometimes inquiries made about substituting ammonium sulfate for ATS in blending with UAN solution. Ammonium thiosulfate is a preferred source of sulfur in clear solution fertilizers because it allows for much higher nutrient analysis in finished blends than ammonium sulfate. Ammonium sulfate (i.e. 8-0-0-9S solution and 21-0-0-0-24S dry) has not been used much in solution fertilizers in the past because of the lower grades that are possible – especially if potassium chloride is included in the blend.
It is possible to make an 8.7-0-0-10S solution that has a salt-out temperature of about 32°F. However, the common grade of ammonium sulfate solution found in the market is 8-0-0-9S and is commonly used as a water conditioner for herbicide applications. Generally, 32% UAN solution and ammonium sulfate solution are compatible in any proportion. Table 1 provides information on various 8.7-0-0-10S and 32% UAN solutions that have a salt-out temperature of about 32°F. If 28% UAN and/or 8-0-0-9S solution is used, the resulting blends will have a salt-out temperature of less than 32°F.
Table 2 shows several blends made from dry ammonium sulfate (21-0-0-24S), 32% UAN solution and water. For those with limited experience in formulating liquid fertilizers, keep in mind that dissolving ammonium sulfate to produce these blends is different than adding ammonium sulfate to your glyphosate herbicide mixes. Efficiently solubilizing dry ammonium sulfate in UAN solution and water requires adequate agitation, water/UAN temperature of 50-60°F or greater and adequate equipment.
Tables 3 contains several properties of various ammonium sulfate solution grades while Table 4 details the negative heats of solution of ammonium sulfate, urea, ammonium nitrate and potassium chloride when dissolved in water. The greater the negative heat of solution and the greater the heat requirement to dissolve the various salts in water.