Sulfur fertility status of grain legume systems in smallholder settings of selected regions in Tanzania

Sulfur (S) is essential for legume protein quality but remains poorly quantified in Tanzanian grain-legume systems. This study evaluated soil SO4-S, leaf S at flowering, grain nutrient composition, and N:S ratios across Dodoma, Tabora, Morogoro, and Mbeya. A complementary survey of 82 smallholder fields was conducted, collecting 54 soil + grain samples (Dodoma, Tabora) and 28 soil + leaf samples (Morogoro, Mbeya). Farmer interviews documented management practices. Standard soil and tissue analyses were performed, and correlations identified drivers of grain S content. Simple linear regression (SLR) linked soil S to grain S content and N:S ratio. Soil S ranged from 0.16 to 24.95 mg/kg; Morogoro and Mbeya showed severe deficiencies (57–71% of fields very low/low), while Dodoma and Tabora were generally medium to high. Soil pH was 4.70–7.73, organic C 0.16–0.66%, and total N 0.01–0.07%. Leaf S in Morogoro and Mbeya often fell below the 0.20% critical level. Similarly, mean grain S in Dodoma and Tabora was 0.17 ± 0.03% (Dodoma 0.18 ± 0.03%, Tabora 0.16 ± 0.02%); grain N:S ratios ranged from 15.7 to 47.2. Soil S was the strongest predictor of grain S (SLR R2 = 0.311, p < 0.001). Farmer interviews revealed variable fertilizer use and sowing windows, with early sowing and S inputs aligning with higher leaf and grain S levels. It was concluded that S deficiency is widespread, constraining legume protein quality. Region-specific S fertilization, organic matter build-up, soil pH management, and optimized sowing dates are recommended.