Improved Management of Acid Sulfate Soils for Rice Production in Casamance, Senegal

Casamance is a region in southern Senegal that traditionally produces rainfed rice, but Senegal produces only 1/3 of its rice consumption. Lowland areas, where rice is primarily produced, have acid sulfate soils with low pH and potential aluminum and iron toxicity. The goal of this work was to determine if soil amendments can alleviate soil acidity, counteract the negative biogeochemical effects that occur in flooded conditions, and increase rice yield. A two-year experiment was conducted to test the following soil treatments – agricultural lime, pulverized oyster shell, biochar, and control (no amendment) – in flat and raised beds. Plots amended with lime and shell materials had increased soil pH, base saturation, Ca, and cation exchange capacity. Meanwhile, biochar elevated particulate organic matter and C:N ratios. Exchangeable Fe and Al were negatively correlated with soil pH, while Geobacteraceae populations (Fe reducing bacteria) increased with pH. A greater proportion of the total Fe was strongly bound in fractions that were less bioavailable in plots amended with shell or lime, and overall rice yields were significantly higher following amendment with shell or lime. During the second growing year these effects diminished, suggesting that liming effects did not persist as expected. These results demonstrate the benefits of soil amendments that raise soil pH and suggest that this effect operates by influencing overall soil nutrient availability to rice plants, but further research is needed regarding the timing and sustainability of the beneficial liming effect.