World’s Soil Resources

Box 6.2 | Nutrient balances in urban vegetable production in West African cities
Based on a two year study of urban gardening sites in Niamey (Niger), it was found that N, P and K
balances were all positive, with values for high and low input gardens respectively of 1133 and 290 kg ha -1
for N; 223 and 125 kg ha -1 for P; and 312 and 351 kg ha
-1 for K. Similar N and P balances were reported for
urban vegetable gardens in Kano (Nigeria), Bobo Dioulasso (Burkina Faso) and Sikasso (Mali). However, at
these latter sites, K balances tended to be negative. Overall, urban vegetable production sites appear to be
major nutrient sinks from which large environmental externalities can be expected.
Bouwman, Beusen and Billen (2009) evaluated the impact of four future development scenarios on
nutrient balances for the year 2050. The scenarios, describing contrasting future development in agriculture
nutrient use under changing climate, are based on the Millennium Ecosystem Assessment. In the most
pessimistic case, the global N balance may increase by 50 percent in the coming decades. In case of proactive
policies aiming at closing the nutrient balance, the N balance is expected to remain constant at 150 Tg yr -1 .
Regarding P, all scenarios predict a future increase in global soil P balance. These global balances hide large
variations across regions and even across land uses. Unfertilized rangelands are likely to maintain negative
P balances. Scenarios with a reactive approach to environmental problems portray significant increases in N
and P balances in Asia, Central and South America and Africa, which can be strongly reduced by a proactive
approach. For North America, Europe and Oceania, a shift from reactive to proactive environmental policies
could allow limiting the increase in N and P balances, or even a decrease in the overall nutrient balance.
Whereas large positive nutrient balances sustained for extended periods of time in industrialized countries
have resulted in negative environmental externalities, positive nutrient balances should not be viewed as
necessarily environmentally harmful. Indeed, in many developing regions (e.g. sub-Saharan Africa), positive
P balances are needed to restore soil fertility potential depleted by long lasting nutrient mining and to boost
the often very low crop yields. Inputs of N in organic form may also be beneficial as part of a strategy to restore
the soils’ organic carbon stocks. Possible negative environmental externalities should be weighed against the
benefits of food security, economic welfare and social well-being. To minimize the negative externalities, the
best nutrient management approaches should be promoted through judicious policies.
6.9 | Soil compaction status and trends
Soil compaction is an important problem affecting productivity of soils across the globe. A hidden problem
of soils occurring on or below the surface, compaction impairs the function of the subsoil by impeding
root penetration and water and gaseous exchanges (McGarry and Sharp, 2003). Soil compaction reduces soil
macroporosity e.g. from an optimum of 6 to 17 percent, and hence reduces pasture and crop yield (Drewry,
Cameron and Buchan, 2008).
Soil compaction in most circumstances is a function of soil type (texture, mineralogy, organic matter),
soil-water content and land management (e.g. tillage practices, traffic, grazing intensity). The problem is not
limited to crop land but is also prevalent in rangelands and grazing fields, and even in natural non-disturbed
systems. Soil compaction occurs when compressible soils are subjected to traction e.g. in forest harvesting,
amenity land use, pipeline installation, land restoration, wildlife trampling (Batey, 2009) or winter grazing
(Tracy and Zhang, 2008).
Trampling mechanically disrupts soil aggregates and reduces aggregate stability (Warren et al., 1986) and
its effect increases with stocking intensity (Willatt and Pullar, 1983). The degree of damage associated with
Status of the World’s Soil Resources | Main Report Global soil status, processes and trends
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