Insufficient potassium and sulfur supply threaten the productivity of perennial forage grasses in smallholder farms on tropical sandy soils

Perennial forage grass production has the potential to improve smallholder livelihoods in the tropics. However, nutrient management is often challenging, especially on infertile sandy soils. This study tested whether typical nutrient management limits the productivity and sustainability of perennial forage grass systems on sandy soils. Nutrient balances were estimated for four fields of either Megathyrsus maximus cv. Tanzania or Urochloa hybrid Mulato II. Nutrient limitations were then evaluated in a nutrient omission experiment. All soils were sandy (< 10% clay), neutral to acidic (pH (CaCl2) 4.6 to 6.7) and had plant-limiting concentrations of total nitrogen, extractable potassium and extractable sulfur. Nitrogen inputs were typically higher than outputs, with balances ranging from −16 kg ha−1 yr−1 to 293 kg ha−1 yr−1, yet concentrations in forage shoots were low at all sites. Phosphorus balances ranged from −5 kg ha−1 yr−1 to 77 kg ha−1 yr−1 and concentrations in forage shoots were adequate. Potassium inputs were low, resulting in balances from −79 kg ha−1 yr−1 to −138 kg ha−1 yr−1 at locations that did not apply inorganic potassium fertilizer. Potassium concentrations in forage shoots were low and omission of potassium resulted in severely depressed biomass production. Inorganic sulfur fertilizers were not applied to forages and the sulfur balance varied from −1 kg ha−1 yr−1 to −24 kg ha−1 yr−1. Sulfur concentrations in shoots were correspondingly low and production was depressed when sulfur was omitted in the experiment. Balanced nutrition was not achieved, despite substantial fertilizer inputs, resulting in deficiencies of potassium and sulfur, inefficient use of nitrogen and excesses of phosphorus. If current practices continue, potassium and sulfur depletion, phosphorus accumulation and soil acidification can be expected. Recommendations for balanced nutrient management that accounts for high rates of removal in biomass, variable concentrations in organic fertilizers, and leaching potential, are needed to sustain the productivity of perennial forages on tropical sandy soils.