World’s Soil Resources

Within the same continent, large variations in nutrient balances may occur. For 13 African countries,
estimated balanced or negative nutrient budgets for N, P and K. At the national level, estimated soil nutrient
balances for the year 2000 ranged from -2 to -60 kg N ha -1 yr -1 , from 0 to -1 1 kg P ha -1 yr -1 , and from -2 to -61
kg K ha -1 yr -1 . A later study at 1 km² resolution confirmed the overall negative balances but highlighted larger
variability over short distances. The rate of nutrient mining by crops was generally low or moderate, because
of low land productivity (low yields), but accumulated over many decennia nutrient depletion may become
severe and may be strongly aggravated by soil erosion.
Based on a review of 57 nutrient budget studies related to the African continent, confirmed that N budgets
at field and farm scale were largely negative whereas for phosphorus negative balances were reported in only
56 percent of the studies. Going from the continental scale to the plot scale, there was a tendency for the
variability in nutrient budgets to increase. This is to be expected, as land uses and management practices in
smallholder agriculture in Africa are highly diversified between farms, within farms and even within plots. The
study did not find a clear trend in the magnitude of the nutrient budgets from plot to continental scales. This
is in contrast to other studies which did report increasingly negative balances as the scale increased.
Box 6.1 | Livestock-related budgets within village territories in Western Niger
(Schlecht et al., 2004)
In the Sahelian zone of West Africa, between 1.5 and 9 kg N ha -1 yr -1 and between 0.06 and 0.7 kg P ha -1
yr -1 are taken in by grazing livestock. The quantity varies by location and land use type (rangeland,
cropland, fallow). However, up to 95 percent of the nutrients consumed by livestock are recycled through
faeces. About 40-50 percent of these faeces end up being spatially concentrated at corralling spots or
in farmyards, which represent only a few percent of the total village lands. Though nutrient in- and outflows
related to livestock account for only a small fraction of the nutrient flows in Sahelian crop-livestock
systems, livestock thus plays a major role in the spatial redistribution of nutrients. Negative balances
occur on rangelands and variable (positive or negative) balances are found in croplands depending on the
intensity of application of organic amendments.
At even smaller scales, differences in soil fertility may arise from differential nutrient budgets. Strong
gradients in soil fertility have been reported around villages, compounds, trees and shrubs as a result of
higher levels of inputs (litter, household refuse, human excreta, manure and urine from resting animals,
sedimentation, etc.) near these features. These are referred to as ‘fertility rings’ or ‘fertility islands’.
6.8.4 | Nutrient budgets: a matter of land use system, land use type, managementand
household equity
Nutrient balances vary greatly across land use (LU) systems. Intensive growing of industrial crops in Europe
is generally characterized by excess inputs of N, despite a recent tendency towards reduced fertilization
driven by EU regulations and the economics of fertilizer use. As a result of the decoupling of livestock and land
and because livestock are increasingly fed with imported feed, pastures are commonly exposed to excessive
applications of manure (e.g. in Normandy in France, and in Denmark and Holland). Regarding P, after decades
of excess application of P, there is nowadays a tendency for farmers to reduce their P application rates, or even
to stop applying P altogether and to rely only on accumulated soil reserves and P released from soil mineral
weathering.
At the other extreme, subsistence farming in developing countries is commonly characterized by negative
balances, reflecting nutrient mining (Roy et al., 2003). examined nutrient balances for different land uses in
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