Description

Increasing agricultural productivity is essential to meet the rapidly increasing demand for food in Sub-Saharan Africa (SSA). According to statistics from the FAO, while the population of the region tripled between 1980 and 2020, the productivity per unit area of sorghum, the main grain of semi-arid regions in SSA, increased by only 20% and remains stagnant. To address this problem, West Africa is currently redeveloping its cultivation guidelines, which take account of agro-ecological zones reflecting climate, but not soil type differences. However, a special soil type called Plinthosols, in which the effective soil depth (ESD) is less than 50 cm, is widely distributed in the semi-arid regions of West Africa. Because these soils have lower water-holding capacity than other soil types, the response of sorghum to fertilizer application may differ on the Plinthosols. 

Therefore, this study aims to determine the differences in fertilizer response of sorghum on three dominant soil types in semi-arid West Africa: Lixisol (LX), which has a thick ESD of about 100 cm and high water-holding capacity; Plinthosol (PT), which has an ESD of about 50 cm; and Plinthosol (PX), which has an ESD of about 25 cm (Fig. 1). 
In a year with 21% (1.3 times the standard deviation) less rainfall than the average year, yields are not reduced in LX, but are reduced in PT and PX (Table 1A). This indicates that lack of soil moisture can limit yield in any of the Plinthosols, suggesting that the optimal sorghum variety (e.g., earlier maturing) and sowing density (e.g., more sparsely planted) may be different in Plinthosols than in Lixisols. The optimal nitrogen (N) application rate for sorghum is 74 kgN ha−1 in LX and PT but 37 kgN ha−1 in PX. The reason for this probably is that the PX with 25-cm ESD has a very limited water-holding capacity and is unable to meet the increased water requirements that accompany the vigorous growth of sorghum with fertilizer application. Since the fertilizer response of sorghum varies greatly among LX, PT, and PX, it is necessary to distinguish between LX, PT, and PX and consider optimal fertilizer amounts, varieties, and seeding densities in the guidelines currently being redeveloped for cultivation in West Africa. This would pave the way for the development of tailor-made cultivation guidelines that will allow farmers to maximize fertilizer application efficiency. Finally, we would like to mention that soil types can be easily determined by ground-penetrating radar in semi-arid West Africa. (For more information, refer to Research Highlight A04 in FY 2018, "Ground-penetrating radar can predict the soil depth at which the petroplinthic horizon starts in the Sudan Savanna, West Africa").
 

Figure, table

1.