Comparing various sulfur sources for soybean grown on an acid Oxisol in Brazil

A greenhouse experiment and a 2‐yr field trial were conducted to compare various S sources for soybean [Glycine max (L.) Merr.] grown on an acid Oxisol in Brazil. In the greenhouse, the sources included ammonium sulfate (AMS), monoammonium phosphate with a S ratio = 50:50 of AMS/elemental S (ES) – (AMS + ES), and ES + bentonite containing 90% ES. All sources were surface‐applied at 0, 5, 10, 20, and 40 mg S kg–1. Separately, all S sources were incorporated into the soil at 20 mg S kg–1. Yield and S uptake by the grain followed the order AMS = MAP – (AMS + ES) > ES + bentonite. No significant differences in grain yield and S uptake were observed between surface application and incorporation, except those of surface application were greater than those of incorporation with MAP – (AMS + ES). In the field trial, single superphosphate (SSP) and phosphogypsum (PG) were also included. All sources were surface‐applied at 30 kg S ha–1 yr–1. Additionally, AMS was split‐applied in equal amounts at planting and R1 stage. In both years, grain yield and S uptake followed the order AMS = MAP – (AMS + ES) = PG = SSP > ES + bentonite = control. The highest S uptake was observed with split‐application of AMS in the first but not the second year. The benefit of a split application of AMS over a one‐time application for S uptake in the first year was probably caused by reduced SO4–S leaching. Core Ideas No oxidation of elemental S with bentonite provided available S for soybean growth. The various forms of S sources differed in S availability for soybean growth. Split application of ammonium sulfate was more effective than one application at planting. Different S sources showed the same the internal S efficiency in mature soybean.