INSET ONE
DEFINITION OF TERMS
Drylands refer to areas where there are desert-like conditions and include hyper-arid through to drysubhumid areas. Rainfall is distinctly seasonal and there is a negative balance between precipitation and
evapotranspiration.
Land degradation is defined as a reduction in the productivity of agricultural land and can involve
deterioration of water resources, vegetation or soils.
Desertification is land degradation in drylands.
Drought is often cited as the cause of desertification and has many definitions all revolving around
reductions in mean annual rainfall. Droughts are a common phenomenon in drylands where low annual
rainfall is coupled with very high inter-annual variability.
INSET TWO
CAUSES AND PROCESSES OF DESERTIFICATION
Farming in dryland areas will almost always lead to accelerated soil erosion. Removal of vegetation cover
in preparation for planting or through grazing, coupled with soil disturbance, increases soil erodibility and
the likelihood of wind and water erosion. This can become a problem if it results in a decline in soil
quality. Erosion processes are selective, in that the finer and organic rich particles are most easily
removed. This can significantly reduce soil nutrient levels and adversely affect soil structure. The residual
coarser soils is less able to retain moisture and is therefore more susceptible to drought. Organic deficient
soils are also less able to retain nutrients for crop growth. Dust storms may become a problem in areas
where there is a significant amount of wind erosion. This is not only a nuisance for local people but can
seriously damage crops through grain impacts. Deposited sediment can also smother young crops and
affect water supplies.
There are also non-erosive degradation processes associated with agriculture. Salinisation occurs when
salts concentrate in the topsoil as evaporative pressures draw moisture to the surface. It is often associated
with irrigation where there is a surplus of water from which the salts are derived and soils become too salty
for crops to grow. Acidification is common in dryland soils where inorganic fertiliser has been applied. If
crops can not use all the nitrogen, increased ammonium (NH 4+) concentrations will cause acidification as
oxidation releases nitrate (NO3-) and free hydrogen (H+) ions, which are responsible for increased acidity.
This will greatly reduce crop yields. Bush encroachment is a problem affecting many dryland areas used
for grazing. Indigenous woody shrubs and trees become the dominant vegetation cover on former
grassland areas as a result of intensive grazing. Fewer palatable grass species are available for animals and
therefore land productivity is reduced.