Is Sulfur Limiting Soybean Yield In New York?

Abstract

In New York, soybean (Glycine max (L.) Merr.) yields have not yet reached their genetic potential. Reduced ambient sulfur (S) deposition, combined with increased crop yields over time and a change from S-containing nitrogen (N) and phosphorus (P) fertilizers to urea, monoammonium phosphate (MAP) and diammonium phosphate (DAP) which do not contain S, has contributed to increased occurrence of S deficiencies in forage and grain crop such as alfalfa (Medicago sativa L.) and corn (Zea mays L.) in recent years. Given a reduction in S deposition from an estimated 10 kg SO4-S in 1991 to 3 kg SO4- S ha-1 currently, combined with an increase in yield over time from 2,690 kg ha-1 to greater than 4,708 kg ha-1, farmers are now asking the question if S could be limiting soybean production in New York. Sulfur is especially important for legumes like soybeans as it plays a critical role in N2 fixation. Plants use S to regulate photosynthesis and build proteins and enzymes. In soybean plants there are two key amino acids that contain S, cysteine and methionine. Insufficient S supply can result in a reduction of both yield and crude protein (CP) content in soybeans, impacting crop production and grain quality. Thus, it is important to ensure sufficient S is available, especially given the increasing yield potential of soybeans. Chapter 1 summarizes the literature on S needs for soybeans. In particular, we identify historical trends in soybean production versus decline in atmospheric S deposition, discuss factors affecting soybean supply of S for growth, yield and nutritional value, and present studies on tools used for S management. Modern soybean varieties now produce more biomass (photosynthetic activity) with improved nutritional value thanks to a combination of breeding and agronomic management improvements. This translates into higher S uptake per cropland unit. Reduced S supply by deposition combined with greater S removal in harvest explains the increasing incidence of S crop deficiencies and, thus, the need for tools to assist in S management decisions. Tissue testing can be used to assess S deficiencies of plants. Soil S testing can identify potential for crop response to the application of S fertilizer earlier in the season but additional work is required to establish critical values for soybean soil testing methods. Field S balances can also be viewed as a management method for end-of-season assessment. Chapter 2 presents the result of on-farm S response trials, conducted to understand the impact of S fertilization on soybean grain yield, crude protein (CP), S content, N:S ratio, and crop S balances. On-farm, replicated, trials were performed in thirteen locations in western New York comparing two S sources (CaSO4 and (NH4)2SO4) versus a no-S control. The lack of response to S addition in tissue analyzes conducted at early bloom and full maturity were consistent with results of soybean grain yields and CP levels, and in N:S ratios and did not suggest an S limitation. The negative partial S balances of the control treatments (crop removal – S supply by S deposition) showed a significant contribution of soil S to total crop supply. Soil organic matter (SOM) measured by the loss-on-ignition, however, was not a good soil S supply predictor. Therefore, critical soil test S levels could not be determined given none of the thirteen trials showed a crop response to the addition of S. We conclude that S at this time does not limit yield potential but continued monitoring is important as partial balances indicate that S may become deficient over time.