Understanding yield-limiting factors for sorghum in semi-arid sub-Saharan Africa: beyond soil nutrient deficiency

Even in semi-arid regions of sub-Saharan Africa, low crop yields are often attributed to a lack of soil nutrients and limited use of chemical fertilizers. However, in areas where soils with low effective soil depth are common, water availability may be a more significant factor that limits crop production than a lack of soil nutrients. In Sudan Savanna, the effective soil depths of three dominant soils, namely Pisoplinthic Petric Plinthosols (PT-pt.px), Petric Plinthosols (PT-pt), and Ferric Lixisols (LX-fr), are < 30, 30-50, and similar to 100 cm, respectively. Consequently, a lack of soil water is anticipated for both Plinthosols types. This study investigated the limiting factor in sorghum cultivation for each dominant soil in Sudan Savanna by creating plots with and without irrigation and fertilization. The results indicated that neither rainfall nor irrigation significantly affected sorghum yield under unfertilized conditions in all soil types, suggesting that even in very shallow PT-pt.px, a lack of soil water was not the limiting factor under unfertilized conditions. However, under fertilized conditions, reduced rainfall significantly decreased sorghum yield in PT-pt and PT-pt.px. In conclusion, in addition to the lack of soil nutrients being the limiting factor in sorghum cultivation under unfertilized conditions, regardless of soil type, lack of soil water becomes a limiting factor in soils with an effective soil depth of < 50 cm (e.g., Plinthosols) under fertilized conditions. These findings underscore the importance of knowing the effective soil depth when developing efficient fertilizer application methods to achieve food security in semi-arid regions of sub-Saharan Africa.