Integrated Soil Fertility Management (ISFM) represents a holistic approach to maintaining and enhancing soil productivity in smallholder farming systems through the combined use of organic and inorganic fertilizers, improved crop varieties, and sustainable agricultural practices. This study examines the implementation and effectiveness of ISFM practices across 150 smallholder farms in sub-Saharan Africa over a three-year period (2021-2024). The research employed a mixed-methods approach, combining field experiments, farmer surveys, and soil analysis to evaluate the impact of ISFM on crop yields, soil health indicators, and economic sustainability. Results demonstrate that farms implementing comprehensive ISFM practices achieved 35-60% higher crop yields compared to conventional farming methods, while improving soil organic carbon by 15-25% and reducing fertilizer costs by 20-30%. Soil pH increased from 5.2 to 6.1, and available phosphorus content improved by 40%. Economic analysis revealed a benefit-cost ratio of 2.3:1 for ISFM adoption. The study concludes that ISFM offers a viable pathway for sustainable intensification of smallholder agriculture, contributing to food security while maintaining environmental integrity. However, successful implementation requires supportive policies, access to quality inputs, and farmer education programs.