There is a downward spiral of declining soil fertility, low crop yield and increasing poverty in the less-favored areas of SSA. The semi-arid tropics of northern Ghana share this episode. The soils in this part of the country are naturally less endowed, have little organic matter content and farmers use very little inorganic fertilizer. Existing studies indicate that the erratic nature of rainfall in the area increases risk and constrains farmers' investment on inorganic fertilizer. However, agronomic studies suggest that promotion of sustainable use of inorganic fertilizer is indispensable at least in the short to medium term to break the downward spiral. Therefore, promoting sustainable use of inorganic fertilizer use remains to be a policy challenge. This paper argues that in spite of observed disinvestment on irrigation both by governments and donors there is significant complementarity between irrigation and inorganic fertilizer use in the less-favored areas of northern Ghana. This implies that increased irrigation investment in the semi-arid tropics of SSA can be justified given its importance in reducing rainfall risk and boosting inorganic fertilizer use.