Seven main groups
of land degradation processes are normally distinguished.
Mass movement is the movement of soil and/or rock downslope,
under the influence of gravity, without necessarily being influenced by water
or ice. Nevertheless, water or ice may make mass movement even more
catastrophic. Mass movement classifications based on the material (mud, soil,
earth, rock and debris), or movement type (falls, topples, slides, lateral
spreads and flows)1.
Water erosion is the removal of topsoil (up to 20 cm) due to
the action of water. It is divided into the following 3 types based on
increasing severity: Sheet erosion (water moves horizontally over a large area),
rill erosion (small incisions take place as the water begins to collect
along parts of the land at lower elevation) and gully erosion (entire sections
of the soil can be washed off)2.
Wind erosion occurs when strong winds blow over
light-textured soils that have been heavily grazed during periods of drought3.
Wind erosion has many impacts. Firstly, fertility is reduced since the majority of the plant
nutrients are concentrated on the soil that is blown away. This reduces the
soils capacity to support productive pastures and sustain biodiversity. Secondly,
it makes difficult the revegetation of the land, since there is a lack of
nutritious soil for the plants to be planted on.
Salinity in agricultural terms is the
excess of salts above the required plant level. Most often it poses constrains
in the growth and productivity of the plants and, therefore, it is a serious
concern. Mingling with other environmental factors as precipitation,
temperature, flooding, soil profile, water table exaggerates the catastrophe4.
Chemical degradation refers to the accumulation of toxic
chemicals and chemical processes which impact on chemical properties that regulates
life processes in the soil5.
? change in one or more of these soil chemical properties has a direct and
indirect adverse effect on the chemical fertility of soils. Chemically degraded
soils have the presence of large amounts of toxic chemicals interfering with
activities of soil life processes. These toxic chemicals may also interfere
with nutrient availability. Chemical soil degradation is hard to spot and thus
may be overlooked.
Physical soil fertility is the ability of the soil to enable the flow and storage of water and air into it, to permit root growth and to
anchor the plants. To be fertile a soil needs abundant and interconnected pore
space. Pore space refers to the volume of soil voids that can be filled by water and/or air. Pore space generally depends on aggregates
of soil particles held together by soil organic matter. Unfortunately,
intensive or inappropriate tillage practices have been a major contributor to
land degradation. Soil tillage breaks down aggregates,
decomposes soil organic matter, breaks pore continuity and forms hard pans
which restrict water and air movement and root growth. On the soil surface, the
powdered soil is more prone to sealing, crusting and erosion. Improving soil
physical fertility involves reducing soil tillage to
a minimum and increasing soil organic matter.
Soil biological fertility refers to the quantity and
diversity of soil flora (all the plant life) and fauna (all the animal life) present in the soil6.
Biological activity is necessary to break down crop residues (roots, etc.) into humus (the organic component of soil). Soil fauna (including earthworms, termites, insects, etc.)
also transfer crop residues into the soil, increase soil porosity and pore
continuity, and can help break down compacted layers. That means that a
constant food source is necessary to maintain soil fauna and flora. A bare soil
means low levels of biological activity. Tillage (we mention many of its
disadvantages in the physical degradation) also disrupts the tunnels and
habitat of organisms. The best way to increase soil biological activity is to
get as close as possible to a natural system. That means, stop soil tillage and
leave plant residues as mulch on the surface.