Recent increases in the human population have placed a great strain on the world's soil systems. More that 5.5 billion people are now using about 10 % of the land area of the Earth to raise crops and livestock. When used for such purposes, soils can suffer various types of degradation that can ultimately reduce their ability to produce food resources. The following graph describes the relative importance of the major soil degradation mechanisms.
Source of Data: Oldeman, L.R., R.T.A. Hakkeling, and W.G. Sombroek. 1990. World Map of the Status of Human-Induced Soil Degradation. An Explanatory Note, rev. 2nd edition. International Soil Reference and Information Centre, Wageningen, the Netherlands.
Erosion is the number one factor degrading soils globally. Erosion
is a process where wind and water facilitate the movement of
top soil from one place to another. Water erosion is more detrimental to
soils globally both by the volume of soil removed and area of land
influenced. Soils are normally protected from erosion by the above- and
below-ground parts of plants. Above-ground parts of plants, like stems and
leaves, reduce the potential of wind and water to erode soils by acting as
barriers to these mediums. Plants can also reduce erosion by binding and
anchoring soil particles to roots.
Agriculture increases the risk of erosion through its disturbance of
vegetation by way of land-use conversion, tilling or overgrazing. Many
farmers prepare land by tilling or ploughing their fields to produce a
smooth planting surface devoid or
vegetation. This process, however, creates a soil surface that is very
vulnerable to erosion. In Canada and the United States, some farmers have
been using a technique known as conservation tillage or zero
tillage to reduce the erosion problem. This technique uses special
machinery and herbicides to plant crops with minimal disturbance to the
soil surface.
The following agricultural practices lead to accelerated soil erosion:
- Overgrazing of animals (where more animals are raised than the forage can sustain). Trampling and eating diminishes the number of species grown in a particular forage area, and without adequate vegetative cover the land becomes more susceptible to both wind and rain erosion. Further when animals are grazed in riparian areas (areas next to streams), the trampling near the stream banks causes erosion and stream sedimentation.
- Planting of a monoculture. This practice can lead to erosion for several reasons. First, a monoculture is harvested all at one time, which leaves the entire field bare and the natural rainfall is not retained by the soil and flows rapidly over the surface rather than into the ground. Secondly, if a disease or pests invade the area, the entire crop is usually wiped out and again leaving the bare soil susceptible to the elements.
- Row cropping. This agricultural practice is common with monocultures but can also be found in polycultures. This technique exposes the soil between each row of crops which is then vulnerable to erosion.
- Tilling or plowing. This is one of the oldest agricultural practices, it involves mixing up the nutrients within the soil, loosening the soil particles, incorporating oxygen and getting rid of weeds, however, it also increases the likelihood of erosion because it disturbs the natural surface and protective vegetation.
- Crop removal. The continuous removal of crops does not only increase the soil susceptibility to erosion due to exposure but it also increases it because the organic matter in the soil is depleted. Organic matter has the ability to absorb a lot of rainwater and without it, erosion is increased because water doesn't soak into the soil.
- Development of new land. This is a problem particularly in the least developed countries. Rising populations are forcing people onto marginal lands to grow crops. Hillsides are not developed properly, and are very vulnerable to erosion when water passes over them.
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