**Abstract**

Fertilizer is an essential input for wide-scale sustainable intensification of crop productivity in tropical Africa, but its use by smallholders is often financially constrained. Four fertilizer use issues are addressed. Smallholders need high net returns from their investments, with acceptable risk, which can be achieved with good crop-nutrient-rate choices made in consideration of the farmer's financial and agronomic context. Soil acidification, which is affected by crop N supply, is best managed with the use of slightly more acidifying but less costly common N fertilizer, e.g., urea, coupled with lime use compared with the use of more costly but less acidifying N fertilizer such as calcium ammonium nitrate. This chapter addresses the feasibility of tailored fertilizer blends for maximizing farmer profit with respect to the nutrient supply cost, the need for flexibility in nutrient application according to the farmer's context, and the weak justification for tailoring blends based on soil test results. The use of a wellformulated blends is justified in some cases, e.g., for some crops in Rwanda, but the supply of blends does not justify restricting the supply of common fertilizers. Farmers need to be aware that unregulated nontraditional products very often fail to provide the claimed benefits. Fertilizer use, sometimes with timely lime application, can be highly profitable with modest risk with good crop-nutrientrate choices, adequate free-market fertilizer supply, and avoiding products with unsubstantiated claims.

**Keywords:** Africa, smallholder, fertilizer, profit, blends, soil acidity, non-traditional products

#### **1. Introduction**

Fertilizer use is essential for widespread sustainable increases in crop productivity and for the preservation of the cropland resource base in tropical Africa. Smallholder farmers in tropical Africa generally have severe financial constraints and need high returns to justify an investment, often >100% within a year [1]. Risk needs to be low given the vulnerability of their livelihoods to failed

investments. Fertilizer use can have a high probability of high profit with wellinformed crop-nutrient-rate choices but also with efficient input supply, favorable credit terms, subsidies, and efficient marketing of the commodity produced [2–4]. The objective of this chapter was to explore four issues affecting the profit potential of fertilizer use by financially constrained smallholder farmers: (1) the choice of fertilizer use options with the greatest potential return on investment, (2) the choice of N source and management of soil acidification, (3) the use of tailored fertilizer blends as alternatives to common straight fertilizers, and (4) the alternative nontraditional products for managing soil productivity. The implications for farm profitability are fundamental to the discussion of these issues.
