Can farmers in sub-Saharan Africa (SSA) boost crop yields and improve food availability without using more mineral fertilizer? This question has been at the center of lively debates among the civil society, policy-makers, and in academic editorials.

Proponents of the “yes” answer have put forward the “input reduction” principle of agroecology, i.e. by relying on agrobiodiversity, recycling and better efficiency, agroecological practices such as the use of legumes and manure can increase crop productivity without the need for more mineral fertilizer.

We reviewed decades of scientific literature on nutrient balances in SSA, biological nitrogen fixation of tropical legumes, manure production and use in smallholder farming systems, and the environmental impact of mineral fertilizer. Our analyses show that more mineral fertilizer is needed in SSA for five reasons:

1. the starting point in SSA is that agricultural production is “agroecological” by default, that is, very low mineral fertilizer use, widespread mixed crop-livestock systems and large crop diversity including legumes, but leading to poor soil fertility as a result of widespread soil nutrient mining. The nutrient inputs derived from agroecological processes, as currently harnessed by smallholder farmers in SSA, combined with the current low fertilizer use, cannot compensate for nutrient exports and losses, that mainly occur in the form of crop harvest products, animal removals, leaching, and soil erosion.

2. The nitrogen needs of crops cannot be adequately met solely through biological nitrogen fixation by legumes and recycling of animal manure. Although legumes can contribute substantially to N input to cropping systems, it is unlikely that they alone can sustain agricultural productivity at the high level that is required for food security. Moreover, recycling of N through manure falls short of what is required to sustain the necessary levels of cereal yields.

3. Other nutrients like phosphorus and potassium need to be replaced continuously. It is clear that these P and K requirements for crops are difficult or even impossible to achieve by smallholder farmers, given their current herd sizes and feed resource limitations. Relying solely on biomass transfers in combination with the ongoing depletion of soil P and K might also have implications for the potential of legumes to efficiently fix N;

4. Mineral fertilizers, if used appropriately, cause little harm to the environment. In general, fertilizer use contributes to four negative environmental outcomes. First, direct soil emission from the application of mineral fertilizers is the most important source of N2O emission from agriculture, together with emissions from organic fertilizer. Second, NH 3 volatilization from fertilizer application can have serious consequences for human health, acidification, and eutrophication of ecosystems. Third, the global increase in N and P fertilizer use has led to the eutrophication of water. Fourth, N fertilizers have resulted in global soil acidification that can undermine soil carbon (C) sequestration and climate mitigation efforts. Yet, these negative outcomes do not come from the fertilizers per se, but from their excessive or inappropriate use. In general, improvement of nutrient-use efficiency can be achieved by widely adopting the 4R principles (the right nutrient source at the right rate, right time and right place).

5. Reducing the use of mineral fertilizers would hamper productivity gains and contribute indirectly to agricultural expansion and to deforestation. Even though SSA is generally considered as having abundant land, accounting for about 60% of the world’s uncultivated arable land, this abundance is primarily concentrated in less than 10 countries. Most countries in the region are facing land scarcity. This makes cropland expansion a limited option, emphasizing the need for production intensification on existing cropland.