Response of Soybean to Additional Sulfur

Trial Objective

• Sulfur is an important secondary nutrient for crops. Soybeans need sulfur for general growth and development as well as nodulation and nitrogen fixation.
• Additional sulfur may be needed in soybean production due to increased crop yield potential, less sulfur contaminants in fertilizers, use of less sulfur-containing pesticides, and fewer industrial sulfur emissions.
• The objective of this study was to determine if additional sulfur has a positive impact on soybean yield.

Research Site Details

• A 3.3 maturity group Asgrow® brand soybean product was used in this study.
• Each treatment consisted of 4 rows, at 30-inch spacing, with 150-foot plot length, and each with 3 replications.

— 28% UAN = Liquid urea ammonium nitrate at 28% N

— Urea = Granular urea at 46% N

—  AMS = Ammonium sulfate ((NH4)2SO4) at 24% S / 21% N

—  Epsom salt = Magnesium sulfate (MgSO4) at 13% S / 9.9% Mg

Understanding the Results

• A field with historically decreasing sulfur levels was selected for this study. After planting, a soil test was conducted on the field to determine existing soil sulfur levels. This most recent soil test showed sufficient sulfur levels, and in general, the yield response was proportional to the sulfur level in the field. Our three fields tested at 8.56, 9.80, and 10.11 parts per million (ppm) for sulfur. Generally, sulfur levels are considered low at 0-5 ppm, moderate at 6-8 ppm, and high when greater than 8.0 ppm.
• In this demonstration, no significant differences were found between any of the treatments (p=0.74). 
• There was a slight positive response from the Epsom salt treatment with an average increase of 1.2 bu/acre over the untreated control.
• There was a slight negative response from the nitrogen alone treatments (28% UAN and urea) with an average decrease of 0.8 bu/acre over the untreated control.
• The nitrogen and the sulfur in the AMS seem to have counteracting effects in this trial. Treatments with nitrogen sources alone decreased yield, whereas the Epsom salt treatments with sulfur increased yield. AMS treatments containing both nitrogen and sulfur exhibited an intermediate effect between the two components alone.
• There were no significant differences due to application timing or placement.

What Does This Mean for Your Farm?

• There was no effect on yield due to adding sulfur in this study. There was also no effect on yield due to application timing or placement. This was likely due to sufficient sulfur levels existing in the soils.