Developing soil-test-based sulfur fertilizer recommendations for soybean

Abstract

Declining sulfur (S) inputs from atmospheric deposition, combined with greater S removal by high-yielding crops, have increased concern about S deficiency in soybean [Glycine max (L.) Merr.]. We evaluated soybean yield and leaflet-S concentration response to six fertilizer-S rates (0, 11, 22, 34, 45, and 67 kg S ha⁻¹) across 50 site-years field trials (16 in 2023, 34 in 2024) in Louisiana using a randomized complete block design with four to five replications. Positive yield responses to fertilizer-S were identified at 11 site-years with 5%–18% yield increase. Control-plot Mehlich-3 soil-test S (STS) at 0- to 15-cm depth explained 53% of relative yield variability, compared to 52% at 0- to 30-cm depth. Critical STS levels were determined as 9.1–10.6 mg kg⁻¹ at 0- to 15-cm depth and 7.5–8.8 mg kg⁻¹ at 0- to 30-cm depth, correctly identifying S-deficient site-years with 83% and 100% accuracy, respectively. However, the accuracy of predicting S-sufficient site-years was lower at 0- to 30-cm depth, with an 11% error rate, compared to 6% at 0- to 15-cm depth. Control-plot leaflet-S concentrations measured as complementary in-season diagnostic at full-flowering, explained 39% of relative yield variation, with a critical concentration range of 2.6–3.0 g kg⁻¹. Site-years with leaflet-S concentrations below this range also had STS concentrations below the critical threshold at 0- to 15-cm depth. A multiple regression model suggests applying 34 kg fertilizer-S ha⁻¹ when STS < 6 mg kg⁻¹ and 22 kg fertilizer-S ha⁻¹ when STS ranges 6–10 mg kg⁻¹ at 0- to 15-cm depth. These recommendations are most applicable to coarse-textured, low organic matter soils in Mid-South.