Legume-Based Integrated Soil Fertility Management Enhances Soil Health and Cotton Productivity in Tanzania

Abstract

Declining soil fertility and nutrient depletion remain major constraints to cotton production in many smallholder farming systems. Strategies that improve nutrient cycling and soil quality while reducing dependence on mineral fertilisers are increasingly receiving attention within integrated soil fertility management frameworks. This study assessed changes in soil physical and chemical properties following the incorporation of velvet bean (Mucuna pruriens), lablab (Lablab purpureus), cowpea (Vigna unguiculata), farmyard manure (FYM), and mineral fertiliser (NPK), evaluated their effects on cotton growth and productivity, and examined residual soil fertility after harvest. Field experiments were conducted from September 2025 to May 2026 at Ukiriguru and Mwamala, Tanzania, using a randomised complete block design with four replications. Soil samples were collected before incorporation, two weeks after incorporation, and after cotton harvest, while cotton growth, biomass accumulation, and yield components were monitored throughout the growing season. Baseline soils were characterised by low organic carbon, nitrogen, sulfur, boron, and zinc contents. Incorporation of legume residues and FYM improved soil organic carbon, nitrogen, pH, exchangeable bases, and micronutrient availability. After harvest, FYM increased organic carbon to 2.53%, while legume-based treatments maintained medium nitrogen status across sites. Seed cotton yield ranged from 872 kg ha-1 in the control to 1,636 kg ha-1 under NPK, with velvet bean and cowpea producing 1,371 and 1,265 kg ha-1, respectively. These findings show that legume residues can improve soil fertility and cotton productivity. Legume cover crops, particularly velvet bean, are recommended to improve soil fertility and sustain cotton production, with further multi-season studies recommended to confirm their long-term effects.