Integrated Soil Fertility Management (ISFM) is defined as “a set of practices, necessarily involving the use of fertilizers, organic inputs and improved germplasm, combined with knowledge of how to adapt these practices to local conditions, aimed at maximizing the agronomic use efficiency of applied nutrients and improving crop productivity. (Vanlauwe et al., 2010).
ISFM was developed in Africa, for Africa, in large part to promote an African Green Revolution based on intensification – increased productivity per unit of land area. Fertilizer use on the African continent is low and fertilizer costs are high compared to the rest of the world due to supply chain inefficiencies.
Fertilizer use is a key aspect of ISFM. Nutrients must be added to a cropping system to compensate for nutrient removal in the form of harvested produce. ISFM is driven by the efficient use of applied nutrients, which not only maximizes the value of fertilizers, but also minimizes nutrient loss to the environment.
The choice of fertilizer plays a key role in nutrient use efficiency. Soils in Africa experience multiple nutrient deficiencies. While nitrogen (N), phosphorus (P) and potassium (K) receive the most attention as they are fertilizer macronutrients used by crops in the largest quantities, large swaths of African soils are deficient in sulfur, zinc and boron, and other secondary and micronutrients (https://www.isda-africa.com/isdasoil/). When nutrients are deficient in a system, productivity is limited by the most limiting nutrient, and when multiple nutrients are deficient, all deficiencies must be addressed to achieve optimal nutrient use efficiency. Balanced fertilizers are those designed to address nutrient deficiencies for a specific crop and soil. Crop- and soil-specific fertilizers are increasingly being produced in Africa, but still account for a small share of the market compared to commodity N, NP, and NPK fertilizers.
Nitrogen losses from nitrogenous fertilizers, particularly urea, are a global environmental concern. Losses include nitrous oxide and ammonia, which are potent greenhouse gases (GHGs). Losses can be greatly reduced by adding urease and nitrification inhibitors to urea- and ammonium-based fertilizers. While such products are available in Africa, they are not yet widely used and offer a positive way to increase fertilizer use efficiency while reducing the GHG footprint of nitrogen fertilizers.
Three factors have been identified as key to achieving efficient fertilizer use: improved germplasm, organic inputs, and local adaptation.
Improved Germplasm
Improved germplasm was a key component of Green Revolution technologies, along with fertilizers, pesticides, and irrigation. Improved germplasm usually means crop varieties bred for disease resistance, high yields, and the ability to use fertilizer efficiently.
Organic Inputs
Organic inputs work in conjunction with fertilizers and are a necessary component of ISFM. Organic matter plays multiple roles in soil health. Soil organic matter increases cation exchange capacity, water-holding capacity, improves soil structure, increases water infiltration, and hosts soil microbes that are beneficial to soil and plant health. When applied to the soil surface, organic mulches reduce erosion, increase water infiltration, and reduce soil temperature and evaporation losses.
Organic inputs can be a challenging aspect of ISFM. The word “input” typically implies an organic source that is brought into the field from an external source, such as compost or farmyard manure. The quantities of such inputs are often limiting to the point that their use cannot be extended to large portions of farmer fields. The term “organic resources” is a better fit for the ISFM definition. Organic resources include crop residues, whether grown in rotations, intercrops and relay crops, with legumes that fix biological nitrogen particularly valuable in the ISFM context. Practically, organic resources that are grown on the field or in proximity are more likely to be adopted on a large scale. A typical maize-soybean rotation, long practiced in commercial farming worldwide, is an example of an ISFM system using in situ-produced organic matter. Combined with minimum tillage and residue retention, this is one of the most adopted ISFM systems worldwide, though not explicitly developed under the ISFM paradigm. Biological N fixed by the soybean is available for the subsequent maize crop, reducing N fertilizer requirement and resulting in a high nutrient-efficient system.
Organic resources at the farm level have multiple uses and are rarely wasted. A common use is as a feed source for farm animals. Other uses include fuel and building materials. Some farmers compost organic residues and use them as organic fertilizer. Because organic residues, compost, and manure are a limited resource, farmers are likely to use them on crops where the financial returns are greatest, which are typically high-value crops such as vegetables. In developing ISFM strategies, one must first understand farmers’ priorities for organic resource use. Otherwise, one may develop ISFM interventions that are poorly adopted because farmers have different priorities for organic resource allocation.
Local Adaptation
Local adaptation recognizes the diverse circumstances of African agriculture and the need to adapt to them. Primary cropping system, livestock intensity, prevailing climate and climate uncertainty, access to irrigation, and market access to inputs and outputs all play key roles in the implementation of ISFM.
ISFM is a set of principles, not a specific cropping system. ISFM principles are designed to intensify production, and as such, access to both input markets (seeds, fertilizers, and crop protection products) and output markets (to sell products and generate income to invest in crop production) are necessary conditions for ISFM adoption.
Next-Generation ISFM
The next generation of ISFM will build on the successes of previous implementations and will include evolving fertilizer technologies, a deeper understanding and consistent implementation of the biological component of ISFM, and better local adaptation based on a better understanding of farmers’ priorities.
Currently, there are over 100 fertilizer blending facilities in sub-Saharan Africa capable of blending multi-nutrient balanced fertilizers. In Rwanda, balanced fertilizers containing S, Zn, B and copper (Cu) were found to be twice as efficient as commodity fertilizers (diammonium phosphate and NPK 17-17-17) when applied to maize and rice, respectively. This represents a doubling of agronomic efficiency. While such robust responses may not be possible across the continent, significant efficiency gains can be achieved with balanced fertilizers in most agroecologies. More efficient forms of urea (urea briquettes and slow-release urea) improve the N efficiency of applied urea by 20-30%. The message is simple: switching to better fertilizers can have a significant impact on agronomic efficiency.
Regarding the organic component of ISFM, the next generation of ISFM must look beyond organic inputs to the management of organic resources. The interpretation of “inputs” as materials applied to fields has led to an overemphasis on animal manure, compost and vermicompost, and recycling of household waste. While these organic resources should be (and usually are) used, they are limited in supply and can only be applied to a relatively small fraction of the land used for agricultural production. From a systems perspective, these inputs do not add organic matter to the system, but rather recycle organic matter already produced as crop residues or pasture. Livestock produce much more organic matter than they leave as manure.
ISFM systems that rely on in situ production and management of organic matter are more likely to be widely adopted. Vanlauwe et al. (2010) describe two such systems: a maize-soybean rotation that uses biological N fixation from the soybean to offset maize N requirements, and microdosed fertilizer systems for sorghum and millet combined with water harvesting and crop residue retention. A similar system is a maize-pigeonpea relay crop.
Finally, the next generation of ISFM must take into account farmers’ priorities. ISFM is an intensification strategy for farmers who are linked to input and output markets, and as such, farmers are more likely to make investments that bring them the greatest returns. Much ISFM research has focused on staple crops, but these are often not the crops that give farmers the greatest return on their investment in fertilizer, organic inputs, and other inputs. Gathering information through socio-economic surveys is critical to understanding farmers’ opportunities and priorities and working with them to design systems for adoption.