The foundation of sustainable agriculture is soil fertility, a term that encompasses:
- the soil’s nutrient content,
- the amount of soil organic matter (SOM),
- the soil structure,
- its pH balance, and
- the presence of microorganisms.
Most soils lack at least some plant nutrients, and growing crops remove nutrients from the soil. Soil nutrients must be replenished when removed and supplied when deficient.
Traditional methods for restoring soil fertility, which entail long fallow periods and shifting agriculture into new areas, may contribute to deforestation, given current pressures on land use. Although historically relevant, these practices no longer suffice to meet the needs of the current and future population levels.
Organic sources of nutrients
Organic sources of nutrients provide fewer nutrients than most people think (and these are not necessarily in plant-available forms), but they are usually excellent for improving other aspects of soil fertility.
The main nutrients in most fertilizers — nitrogen, phosphorus and potassium — come from the land and the air. They exist in crop residues, manure and waste but their plant available forms are not abundant enough for the level of agriculture needed to produce food, feed, fibre and bioenergy to more than 7 billion people.
Inorganic sources of nutrients
Inorganic sources ("manufactured" fertilizers) contain only plant-available nutrients and therefore have no direct influence on soil structure or on the presence of microorganisms.
This is why agronomists generally consider it optimal to use both organic and inorganic sources together, a technique called Integrated Plant Nutrition Management (IPNM).
Erosion and desertification
When soils are not replenished after every harvest with the necessary micro- and macronutrients it needs, it becomes gradually depleted and less fertile, thus more prone to environmental problems, such as erosion and desertification.
Whereas in the developed world, excess application of fertilizer and manure is sometimes blamed for damage to the environment, the low use of inorganic fertilizer is one of the main causes for environmental degradation in Africa.
Bationo, Alliance for a Green Revolution in Africa (AGRA)
Desertification is the degradation of land in arid, semi-arid and dry sub-humid areas, primarily caused by human activities and climatic variations. Although other regions, mostly in Asia and Latin America, are also concerned, the most dramatic examples of desertification are found in Africa, where declining soil fertility is linked to declining agricultural productivity.
Managing soil fertility helps in several ways. High levels of soil organic matter improve water retention. Furthermore, fertile soils can support a more vigorous crop cover, which can help prevent erosion that contributes to desertification.
Improving land productivity is necessary to manage soils and the land. By enhancing farmers' access to improved technology and inputs, especially credit and mineral fertilizers, they can best steward their soils and land and improve their livelihoods.
Farmers will be more successful if they learn to combine locally available organic resources and mineral fertilizers to build up soil fertility and to improve fertilizer use efficiency.
Soil degradation in Africa
Low use of fertilizers is one of the main causes of environmental degradation in Africa. Intensification is needed to feed growing populations, but it must be done in a way that uses soil nutrient and water resources efficiently and that relieves pressure on forests and other fragile lands.
Farmers in sub-Saharan Africa have traditionally cleared land, grown a few crops, then moved on to clear more land, leaving the land fallow to regain its fertility. But population pressure now forces farmers to grow crop after crop, ‘mining’ the soil of nutrients. Africa loses an estimated $4 billion worth of soil nutrients yearly, severely eroding its ability to feed itself. This has led two former World Food Prize Laureates to call this situation a 'catastrophe'.
Olusegun Obasanjo, former President of Nigeria
Solutions need to provide timely and affordable access to agricultural inputs (fertilizers, improved seeds, crop protection products, etc.), credit facilities, infrastructure and market opportunities.
Successful approaches have entailed building farmer capacity by:
- forming cooperatives and teaching business skills,
- credit and insurance schemes for farmers, retailers and wholesalers,
- vouchers to purchase inputs from private-sector dealers,
- providing market and agronomic information through the internet and mobile phones,
- and multi-stakeholder public-private partnerships around specific agricultural development corridors.