Assessing the Effect of Legume-based Fallows on Soil Acidity in Smallholder Agriculture in Sub-Saharan Africa: A Systematic Review

Legume-based fallows have been increasingly investigated for their potential to improve degraded soil health challenges in sub-Saharan Africa. This study evaluated legume fallows’ potential to enhance soil pH, nitrogen, potassium, and phosphorus concentrations in smallholder farming systems. Nine hundred fourteen records were retrieved from three databases, covering academic journals, agricultural studies, and theses/dissertations, and were rigorously screened against the 7 eligibility criteria. The narrative analysis was performed on ten studies while the meta-analysis involved seven studies. Nigeria accounted for (50%) of the included studies, with Cajanus cajan, Tephrosia vogelii, and Pueraria phaseoloides dominant legume species. The results revealed that legume fallows had varied impacts on soil pH, with a small but non-significant impact, SMD = –0.38; p = 0.18). In contrast, nitrogen (SMD = 0.89), phosphorus (SMD = 1.06), and potassium (SMD = 0.56) significantly increased (p = 0.001) in fallows with legumes than non-legume fallow. Subgroup results also showed that soil type, fallow duration, and soil depth influenced outcomes. Arenosols under long-term fallow (>6 years) exhibited greater pH decline, while Alfisols and Nitisols contributed to enhanced nutrient retention in the top layer soil. Findings show that legume fallows enhance nutrient availability but are not consistent on soil acidity. Nonetheless, it is essential to combine other soil amendment strategies such as compost or manure application, or select legume species tailored to soil type and fallow duration to reduce soil acidity.