Impact of effective soil depth on agronomic recommendations for sorghum and its economic viability in Africa

Improving sorghum productivity, the second most important upland crop in Africa, is critical to meeting the continent’s rapidly growing food demand. Previous research indicated that limited soil water, especially in soils with an effective soil depth (ESD) of ≤ 50 cm, restricts sorghum production under fertilized conditions. This finding suggests that optimal management practices, including fertilizer application rates, planting density, and variety selection, should vary by ESD. However, agronomic recommendations for sorghum in Africa have yet to incorporate ESD considerations. This study assessed the impact of ESD on optimal sorghum management practices and their economic stability amid fluctuating fertilizer prices. Two-year field experiments were conducted on soils with ESDs of 25, 50, and 100 cm in the Sudan Savanna, Africa’s largest sorghum-producing region. These ESDs represent three dominant soil types in the region: two types of Plinthosols and Ferric Lixisol. Results showed that the optimal management practices varied by ESD: 37 kg N ha⁻¹, 3.1 hills m⁻², and Kapelga for 25 cm ESD; 74 kg N ha⁻¹, 8.3 hills m⁻², and Kapelga for 50 cm ESD; and 74 kg N ha⁻¹, 5.6 hills m⁻², and Sariaso14 for 100 cm ESD. These practices were economically robust for ESDs of 50 and 100 cm but not for 25 cm. A random forest regression model identified ESD as the most critical factor for sorghum yield, surpassing rainfall, nitrogen application rate, planting density, and variety. This study highlights the need for ESD-based agronomic recommendations to enhance food security and income stability for African farmers.