The Lesson of Drente's 'Essen' Soil Nutrient Depletion in Sub-Saharan Africa and Management Strategies for Soil Replenishment
The term "replenishment" is used in a misleading way when it is suggested that soils are poor through depletion by farmers and that soils should be restored to their original state for agricultural development. This philosophy created awareness for problems confronted by African farmers. It neglects, however, the heterogeneous redistribution of nutrients that is inherent to agricultural land use. Active and passive transport of organic matter causes centripetal concentration of nutrients around farms and villages and maintains or even improves the soil fertility of crucial fields at the cost of surrounding land. The advice to use fertilizers on bush fields in view of the use of compost and manure on compound fields is like “putting the cart before the horse”; the value: cost ratio of using inorganic fertilizer on compound fields is higher than that on bush fields given their negative organic matter and nutrient balances. The integrated use of inorganic fertilizers and organic forms of manure triggers a positive spiral of improved nutrient use efficiency and improved soil organic matter status. The increasing value: cost ratio of fertilizer use improves the access to this and other external inputs. Where crop-livestock integration is an important component of the intensifying production system, the centripetal concentration (see footnote 1) can even turn into the opposite, a centrifugal transport that replenishes (planned or unplanned) the depleted surroundings of farms and villages. Active replenishment of depleted soils is no requirement for agricultural development; intensification can start on village fields where fertility is maintained or improved. However, public investment in soils, focusing on reinforcement of the positive effects of the centripetal concentration of organic matter and nutrients, is recommended; it enables farmers to start fertilizer use where even the compound fields at present do not allow it.
Nutrient use efficiency, Agricultural development, Natural resources