Page 44 - Soil Degradation, Conservation and Remediation

1.7 Soil Degradation 29

and seeds of assorted crops are sown before monsoon. Seeds germinate and grow
rainfed, and the crops are harvested after 6–8 months. Then the land is left fallow.
Farmers clear a new patch of forest for cultivation in the next season. Earlier, they
returned to the previous land for cropping after 15–20 years, but the rotation period
has now alarmingly squeezed to less than 3 years. Overpopulation, settlement of plain
landers to hills, and scarcity of available land for cropping are behind this shrinkage.
This has enhanced deforestation, loss of biodiversity, and soil degradation. It is esti-
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mated that each year approximately 1.9–3.6 × 10 ha land of primary close forests,
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3.4–40 × 10 ha land of secondary close forests, and 6.9–21.9 × 10 ha land of second-
ary open forests are being lost due to shifting cultivation (Detwiler and Hall 1988 ).
Shifting farmers burn the slashed debris so that nutrients in its ash should increase
fertility and enhance crop productivity. The effect of burning is, however, tempo-
rary. Rains following burning remove bases rapidly by erosion and leaching.
Growing evidence suggests that shifting cultivation in its distorted form leads to an
adverse effect on soil and water. Such activities affect soil physical and chemical
properties, reduce nutrient stocks, and accelerate soil erosion and sedimentation.
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Gafur et al. ( 2000 ) estimated soil loss of 43 Mg ha year due to shifting cultiva-
tion in Bandarban Hills, Bangladesh.

1.7.1.3 Overgrazing

According to a UK regulation, overgrazing means grazing land with livestock in
such numbers as to adversely affect the growth, quality, or species composition of
vegetation on that land to a signifi cant degree (Statutory Instrument 1996 ). Wilson
and MacLoad ( 1991 ) include animal performance as well; they state that a grassland
is overgrazed where a concomitant vegetation change and loss of animal productiv-
ity arises from herbivores’ grazing of land. Overgrazing can mean different things
to the grazier and the range manager. For the grazier, it implies that the pasture can
no longer carry as many animals as before, or that its productivity has declined.
Overgrazing is a major cause of soil degradation worldwide (Oldeman et al. 1991 ),
accounting for 35.8 % of all forms of degradation. However, degradation caused by
overgrazing is especially widespread in Australia and Africa, where it accounts for
80.6 % and 49.2 %, respectively, of all soil degradation and least extensive in Europe
(22.7 %) (Warren and Khogali 1992 ).
Overgrazing is the most devastating cause of desertification in arid lands. Livestock

are the main source of income in many arid and semiarid countries. Domestic
stocks are widely diverse and consist of camels, donkeys, horses, cows, sheep, and
goat. The forage and overgrazing of livestock cause a chain of degradation, critically
reducing vegetation cover and soil fertility, as well as increasing erosion. Domestic
animals rapidly clear vegetation, placing stress on a land that already has a low vegeta-
tion cover (Fig. 1.5 ). They also move in large groups and have sharp hooves that easily
break up the soil, leaving it susceptible to erosion. Erosion decreases fertile organic
content of the soil. The lack of organic matter can lead to desertifi cation through
reduced nutrient availability for plant growth.