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Soil mining: myth or reality?

An alarming concept that African soils are being steadily depleted of nutrients due to farming without fertilizers has gained wide credence in the scientific community (Bationo et al. 1998; Buresh et al. 1997; Cleaver and Schreiber 1994; Eswaran et al. 2001; Gruhn et al. 2000; Matlon 1987; Sanchez et al. 1997; Sanders et al. 1996; Smaling et al. 1997; Steiner 1996; Stoorvogel and Smaling 1990; Van der Pol 1992; World Bank-FAO 1996). The narrative of events is as follows.

The risk of drought and consequent crop failures along with financial constraints and low market prices would seem to discourage the poor from investing in their lands (fertilizer, organic matter, fallows etc.) Without these fertility-restoring inputs, the soils become depleted or 'mined' of their native fertility and protective cover (e.g. trees being removed for firewood). As those lands become nonproductive, the poor are left with no choice but to expand onto additional, even poorer lands in order to meet their basic food needs, impoverishing those soils as well. In this way they relentlessly mine until the soil is severely degraded and the people must leave or starve.

Doubts have recently been raised about the generality of this soil mining narrative, though. Evidence for the soil mining hypothesis has mainly come from research-plot data extrapolated to regional and continental scales (de Ridder et al. 2004; Mortimore and Harris 2004; Niemeijer and Mazzucato 2002b). This may be too simplistic, these scientists argue. They point out that it does not account for the wide diversity of smallholder practices and the large number of ecosystem interactions on typical farms. Many scientists have observed that dryland farmers are keenly aware of the importance of maintaining productivity and coping with drought, and are skilled in indigenous techniques for doing so (Dahlberg 2001; Mazzucato and Niemeijer 2000; Mortimore and Harris 2004; Niemeijer and Mazzucato 2002a and 2002b; Reij et al 1996; Scoones et al. 1996; Scoones 2001; Scoones and Toulmin 1998).

Mining, or borrowing?

According to these experts, a more realistic appraisal of soil fertility status in Africa needs to take account of how farmers dynamically manage their soils across their fields and crops, and over the years. A static 'snapshot' from a single field or cropping system won't do, they say.

Animal manure is often collected and carried to the farm plots near the homestead in Africa to enrich the soils that are used to cultivate the highest-value crops (Mazzucato et al. 2001; Scoones 2001). Farmers also apply fertilizer to these homestead plots when they can afford to buy it. Manure collected from livestock and carried in to apply to the homestead fields represents a loss of nutrients from distant grazing lands, but a gain to these more valuable homestead fields. Fertilizer applied to the homestead fields is also a net gain. So the 'mining' effect is often not visible near the homestead; on the contrary, these valued plots are often enriched over time.

Cropped fields further from the homestead, on the other hand typically receive no manure or fertilizer; farmers apply less labor and expect lower crop yields from them. They take an opportunistic strategy with these outer fields. In a good year they may produce a welcome extra harvest, but in a poor low-rainfall year a crop failure does not create an unbearable loss since the investment made was relatively low.

Farmer interviews have shown that they recognize the depletion that is occurring in their outer fields and will correct when it is affordable and remunerative to do so (Scoones 2001; Tiffen 2002). Rural dwellers also shift their village locations over time, altering the positioning of homestead fields versus outer fields. These adaptive responses suggest that farmers are 'borrowing' soil fertility from one area and lending to another, to be paid back later when they are able. But will this ever happen?

As markets develop, for example farmers gain the economic capacity to buy more fertilizer and invest in larger areas of farmland. Farmers also change crops in response to market signals and soil fertility conditions. Thirdly, they seek off-farm employment and send remittances back to the farm-which can include investments in soil fertility such as fertilizer, when economic conditions favor the investment (e.g. for high-value crops).

Viewed in this dynamic context, static estimates of continental nutrient balances from simple experimental plots seem too coarse a tool upon which to interpret farmer behavior or to formulate research, development and policy directions, according to the dissenting scientists.

A fertility borrowing perspective implies a different set of research and development imperatives. Research is needed to evaluate the capacity, resilience, and balance dynamics of these resource pools. How long can the borrowing go on without permanent damage to the outfield and grazing land resources? How can farmers be assisted in their natural inclination to rehabilitate these areas? The fertility borrowing concept highlights the opportunity to build on farmers' responsiveness to markets by introducing infrastructure, policy reform, and technology interventions that reduce the costs of, and increase the rewards from fertilizer use.

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