By Mike McGahuey for Agroforestry World
April 26, 2012
The question is not, “should farmers use inorganic or organic fertilizer?” but rather, “How can farmers increase soil organic matter in a cost-effective way in order to recycle nutrients, increase fertilizer-use efficiency and establish the foundation for building and sustaining soil productivity in Africa?”
Many agricultural developmental agencies either support the idea of farmers using more inorganic fertilizer or very little or none at all. Some scientific literature support the use of inorganic fertilizers while others refute the claim.
However, a review of the scientific literature shows a strong synergy between inorganic fertilizers and soil organic matter. It claims the two can work symbiotically to produce and sustain more productive soils,[i] particularly for resource poor smallholders.
African Agriculture: Dirt Poor by Natasha Gilbert shows how problematic the debate has become.
This is particularly the case on weathered soils– upon which most African dryland farmers depend. [ii] Research out of West Africa by International Fertilizer Development Centre IFDC, Agricultural Research for Development CIRAD, and others show that fertilizer-use efficiency (FUE) is directly and positively related to soil organic matter content (SOM). [iii] These studies show that in weathered soils with low SOM, a high percentage of nutrients applied in inorganic fertilizer are leached past crops’ root zones or are tied up, effectively increasing the unit price and risk to the farmer. Studies in Kenya by Marenya and Barrett[iv] show that farmers know this. Other studies report that farmers preferentially use fertilizers on soils with relatively higher levels of organic manure. [v]
Approaches such as Integrated Soil Fertility Management, in which inorganic fertilizers are applied in tandem with agricultural practices that increase the soil’s organic matter content, have been shown to increase Fertilizer Use Efficiency (FUE) significantly. [vi]
However, lest we think that the solution is easy, IFDC studies point out that amending soil organic matter content is not easy if it means finding tons of organic matter per hectare and then transporting it to the field, if they can even find the 4 or more tons per hectare that many soils would require annually. [vii]
This underscores the convenient fact that agroforestry annually delivers tons of organic manure to the soil. [viii] Consequently, the positive effect of the leaf-fall from Faidherbia albida and other field trees is two-fold—it conveys nutrients from the lower part of the soil profile to a crop’s root zone and it builds up soil organic matter, thereby increasing FUE and establishing the foundation for increasing and sustaining soil productivity.
In light of the evidence, the question is not, “should farmers use inorganic or organic fertilizer?” but rather, “How can farmers increase soil organic matter in a cost-effective way in order to recycle nutrients, increase fertilizer-use efficiency and establish the foundation for building and sustaining soil productivity in Africa?” Part of that answer will be for development institutions to follow the example of many African farmers and to mainstream agroforestry systems within agricultural strategies.
[i] Kelly, Valerie A. (2005). Factors Affecting Demand for Fertilizer in Sub-Saharan Africa. The World Bank, Agriculture and Rural Development Discussion Paper 23.
[ii] Breman, Henk, B. Fofana and A. Mando (2007). The Lessons of Drente’s ‘Essen’: Soil Nutrient Depletion in Sub-Saharan Africa and Management Strategies for Soil Replenishment. In: Braimoh, A.K. & P.L.G. Vlek, 2007. Land use and soil resources. Springer Media B.V., p. 145-166.
[iii] International Fertilizer Development Center (2005). Development and Dissemination of Sustainable Integrated Soil Fertility Management Practices for Smallholder Farmers in Sub-Saharan Africa: Final Report, March 2005.
[iv] Marenya, Paswel P. and, Christopher B. Barrett (2009), Soil quality and fertilizer use rates among smallholder farmers in western Kenya, Agricultural Economics 40 (2009) 561–572.
[v] P Marenya, Paswel P. and, Christopher B. Barrett (2009). State-Conditional Fertilizer Yield Response on Western Kenyan Farms. Amer. J. Agr. Econ. 91(4) (November 2009): 991–1006.
[vi] Wopereis, M.C.S., A. Tame´lokpo, K. Ezui, D. Gnakpe´nou, B. Fofana, H. Breman (2005). Mineral fertilizer management of maize on farmer fields differing in organic inputs in the West African savanna. Field Crops Research 96 (2006) 355–362.
[vii] Breman, 2007.
[viii] Boffa, J.M. (1999). Agroforestry Parklands in Sub-Saharan Africa, FAO Conservation Guide.