Resourceful strategies for sustaining soil nutrient levels

[Summary provided by Plecserv of article by: De Ridder, N., Breman, H., van Keulen, H. and Stomph, T.J. 2004. Revisiting a 'cure against land hunger': soil fertility management and farming systems dynamics in the West African Sahel. Agricultural Systems 80: 109-131. To correspond with the authors, or to ask for a single electronic copy of this paper, write to nico.deridder@pp.dpw.wau.nl]

In the early 1990s some pioneering studies of nutrient budgets for African soils under cultivation indicated serious negative balances, suggesting that soils were being mined of their capacity. This finding corresponded with experimental results showing nutrient losses from soils under continuous cultivation, although sometimes depleting at quite low rates. Despite this evidence and the concern which it aroused, food security in West Africa has improved in recent decades, not only because of expansion of cultivated lands but also because of significant improvements in the yield of cereal crops. Nico de Ridder of Wageningen University, with two of his colleagues and one collaborator resident in West Africa, have recently examined this paradox using evidence from the Sahelian countries, principally Burkina Faso and Mali.

First they note that only a part of the nutrient budget is capable of direct measurement, leaving the effects of some natural processes and the spatial consequences of changes in farming system to be estimated. It would take decades to establish the decline in soil quality experimentally and, given the heterogeneity of soil conditions, it would also require a wholly unreasonable number of samples. The heterogeneity of farms under management and the uneven distribution of manure inputs on the land also militate against sound quantitative determination of nutrient stocks and their trends.

A more effective approach is through the dynamics of farming systems, which undergo rapid change due to population growth, and the development of markets. Old practices depending on long fallows and the livestock of transhumant pastoralists for soil regeneration are giving way to systems in which nutrients are returned to the soil, both by the management of growing numbers of livestock and by forms of composting. An 'infield/outfield' system under which livestock are pastured in the communal outfield but corralled or stabled near or in the village, making much of their manure available for the infield, is common, and can be further intensified by stall feeding with crop residues carried into the homestead. It is mainly by means of such labour-demanding intensification that the improvement in yields recorded since 1960 has been achieved. Examples are presented showing clear gradations of soil nutrient content falling away from the villages into the outfields and beyond.

Use of chemical fertilizer is still limited when viewed on a West African or national scale, but its critical and labour-saving addition is found in two main situations. First is where a cash crop, cotton, has been successfully introduced, and second in areas which are accessible to urban markets. In a province-by-province examination of data from Burkina Faso, the highest cereal yields are not found in regions with the largest density of livestock, but in the higher-rainfall areas where cotton has become important. The areas close to the country’s cities do have relatively high livestock densities, explained by the urban market for meat and milk. But where livestock pressure is heavy there is evidence of range degradation.

Looking to the future, there are clearly limits to the productivity of the indigenous recycling system, and the coming generation of farmers will be faced with the choice between greater use of costly external inputs, and having to rely on migration or other off-farm employment for their living. External inputs are presently affordable only where there is an available market for farm produce. Improvement in infrastructure may make markets more widely available, but there will be increasing need to supplement the indigenous skills which have achieved so much with new innovations. Even though stable soil conditions have been created in limited areas, it is at the cost of continuing degradation in other parts of the landscape from which nutrients are taken. Less and less is available for the pastoralists, who once used the larger part of the Sahel. Although the situation is far less forbidding than it appeared to be to the scientists and policy-makers of around 1990, there is still degradation. [abstract from Plecserv]