Degradation of the soil is caused by natural processes as well as by human activities. Now let us briefly study about both of these. Soil is a complex body and forms the thin outer layer of the earth. It is directly or indirectly developed from the mineral constituents of the rocks. The first phase of this process is weathering which is essentially physico-chemical in nature. Weathering leads to simplification of complex substances forming rocks through disintegration. The second phase is soil formation through the process of consolidation. This is bio-geo-chemical in nature. The process encompasses certain biological influences which result in the synthesis of many substances leading to the development of complex soil bodies with definite physical, chemical and biological properties. This natural process of weathering is slow and is a part of the evolutionary cycle.
The intensity of this natural erosion process can be realized from the rate of denudation or the rate of lowering of land surface. Denudation rates vary widely in India, particularly in the Himalayan belt. For Sutlej basin it is 0.21 mm per year, while it is 20mm per year in Darjeeling. But the general estimate for the earth, as a whole, is between 0.1 to 1.0mm per year. Much of the natural erosion is neutralized by the uplift or rise of land surface. The estimated range is from 1 mm per year to 9mm per year. However, analysis of available information on soil losses^ sedimentation rates and other related data from India, leads to the conclusion that estimated uplift rate is lower than 5 mm per year.
The estimated geological erosion rate in the present times has been at 100cm/1000 years, which is about five times that of the past forty million years. There seem to be no evidence of any unusual geological upheaval in extensive areas in the recent past. Whereas, there has been phenomena! increase in population and attendant activities in the last two or three centuries, which points to the role of man's intervention in aggravating land degradation.
Among all the life forms that have lived and are Living on this earth, human beings have caused serious transformation of natural resource base to subserve various requirements of human society. The interventions are manifold. Often these are intended to obtain maximum from the mother earth, and in the process, have wrought severe disturbances to the balance in the bio-geo-physical systems. These have reduced the earth's capacity to absorb adverse effects and have impaired its regenerative ability to restore the losses suffered through exploitation. On the other : hand, there have been deliberate destructive actions, such as those during the world wide wars which have caused serious damages to the natural defences of this planet. More important interventions are as follows:
Economic activities : Mining, etc.
Developmental works : Settlements, Transport and Communication
We will briefly describe the effects of these on land quality.
Physically the process of deforestation includes repeated lopping, felling, removal of forest litter, browsing and trampling of livestock, fire, etc. Examination of the annual rate of deforestation between 1976 to 1985 reveals that in India annually 0.i?4Mha of forests are being deforested.
A number of studies in India and Nepal have established that the increased demand for food, livestock and firewood is the direct cause of deforestation. These well intended efforts to increase food production are often unsuccessful and usually result in great environmental damage. The consequences of deforestation in terms of soil erosion, land degradation, nutrient loss, and the disruption of the delicate equilibrium among soil, plants and atmosphere can be seen in the vast tracts of barren and unproductive land where lush green forest once grew.
Another important reason for forest conversion is to meet the fuel wood demand. The rural population in the tropics depends almost exclusively on fuel wood as their primary sources of energy. It is estimated that the per cent energy requirements met by fuel wood in rural areas is 90% in Kenya, 88% in Zambia, and 95% each in Nepal, Sri Lanka, and Thailand. The estimated fuel wood consumption ranges from 0.8 to 2.0m3/capita/year, with an average consumption of 1.5 m-1/capita/year. Deforestation will continue unabated unless alternative, equally reliable, and economic sources of fuel are made available to the rural population.
The recent studies carried out by the Indian Institute of Sciences, Bangalore, North East Hill University, Shillong, Centre for Environmental Studies, New Delhi, and others revealed that the increasing rate of deforestation cannot be attributed to the community residing within and around forests and those who depend very significantly on forest produce for their sustenance. In fact, analysis of the traditional flow of forest produce indicates that these people use minor forest produce without damaging the forest cover. On the other hand, with the advent of centralized management of forest resources, much of the community forests were lost to the community and it had to look for alternatives. This may have accelerated exploitation by the community from limited areas accessible to them.
In fact, much of the extensive deforestation is now going on, more to meet off-site resource flow or demands than the unavoidable pressure from the local community to meet their sustenance needs. In place of the traditional rapport between the forests and the community, increasing alienation, or at least indifference, has become widespread. The possible ecological implications of large scale deforestation now taking place in humid and sub-humid tropics has caused much concern among scientists, environmentalists, and planners around the world. In order to assess the environmental changes, local and global impact on climate, and degradation of fragile soil resources, it is necessary that solid data is collected from well-designed and adequately equipped long-term studies, planned to quantify the effects of deforestation.
Agriculture in its multiple dimensions has been a major human intervention to cause soil erosion and loss of bio-diversity. It has interfered with the natural means to regenerate land productivity and replenish available moisture storage. It has also opened the means to contaminate both soil as well as moisture sources with the application of bio-chemicals and irrigation water. The worst form of erosion on farm lands is wash-off or sheet erosion. It is slow and not so spectacular. As described earlier, this is followed by formation of rills due to development of erosive over-land flow and subsequent formation of gullies and ravines. On the arid and semi-arid areas, sand blows and sand shifts act in a similar fashion as sheet erosion does, Where water is the chief agent. Consequently, a creeping effect of desertification sets in and steadily destroys land productivity and its supporting capacity.
Uncontrolled cultivation of mountain slopes without appropriate land treatment measures such as bunding, terracing, trenching, jacketing and rivetting, etc., leads to loss of nutrients. Similarly, due to increasing population, continuous cropping of the same land or enlargement of cultivation over marginal and sub-marginal lands, there is little time for the natural ecosystem to revive and protect the land from erosion. In the areas subject to shifting cultivation, the population pressure shortens the fallow cycle and thus prevents natural regeneration of the multiple plant species as present in a multitiered forest cover. On the other hand, increased number of livestock, particularly goats and sheep, besides stray cattle, have been increasingly grazing the decreasing community lands and other grazing grounds.'One of the most important consequences of this is the increased erosion and decreased growth of plants.
c) Economic Activities
Besides land based communities, minerals and metals as well as fossil fuel are other natural resources that the society needs to meet its energy requirement and for other activities. The extraction of such natural resources demand deforestation and changes in landscape, causing irreversible changes in the natural land resource base. Mined area and mine spoils are a major source of erosion and land degradation with resultant loss of water resources and .and productivity. Mining from greater depth such as for oil, gas, etc., cause subsidence and accompanied erosion.
d) Developmental Works
Human society has many social priorities, such as, shelter, transport, communication, recreation, etc. For all these items, land and land based commodities are needed. Human settlements are a majof threat to the soil and land, as during the construction in the developmental phase, much accelerated erosion takes place. Subsequently, settlements put the land to irreversible non-productive use. Roads, railways, etc. are required to be aligned, cutting across the natural divides such as ridges, valleys, streams and rivers. The construction of such facilities cause voluminous dislocation of earth mass and rock material associated with heavy erosion, disruption of natural drainage system, resulting in accumulation of water at various locations and cause land slides and slips over extensive area.
e) Extent of Soil Erosion
Neither a comprehensive survey for the country as a whole, nor any rapid survey for a reasonable extent of the country, has .been made on the extent of soil erosion. Till 15 years ago, no attempt was made to collect available information from different sources and correlate it with other statistics with a view to assess the magnitude and extent of various soil-related problems in the country. In the early 1970s, the National Commission on Agriculture (NCA), carried out such exercise for the country as a whole and indicated that about 175 Mha or nearly 55% of the country's geographical area is subjected to problems of soil erosion and different types of land degradation.
Doon Valley is an exquisite region full of diverse flora and fauna. It is bounded by Shivalik and Himalayan ranges on one side and Ganga and Yamuna on the other. But, today this scenic valley is in danger of being ruined because of uncontrolled quarrying for limestone, and also because of deforestation. The tree cover in the valley is reported to be only 12% as against officially recommended 60%. Doon Valley is a good example of erosion caused by human activities, such as, mining and deforestation/According to the report commissioned by the Department of Environment, mine debris allowed to fall into river and canal beds, is impeding water flow and affecting irrigation supplies. Further, the limestone belts used to act as a mechanism for capturing, retaining and releasing water perennially. All the important rivers and streams, such as the Yamuna, Bindal, Song, Suswa and Sahashdhara, etc., originate from the same area where the limestone deposits exist. Limestone quarrying has also led to deforestation and loss of grazing lands. Quarrying operations have destroyed forests and natural springs, creating ecological havoc in the area. Mining\operations have loosened the hills, giving way to landslides and silting of the rivers.
Several cost-benefit analysis made by the Government have pointed out that limestone industry would not suffer even if all the operations were shut down in the area. The Government could, therefore, issue an ordinance declaring the Valley as an ecologically fragile zone. In the above case study you have learnt how mining enhances erosion and destroys the land fertility.
We may now study how agricultural activities affect lands, farmers and productivity. A related issue, however, is the effect of agricultural pollution on non-agricultural ecosystems. When the soils are eroded from farmlands, not only the farm but the air is also filled with dust; silt may collect in beds of streams. In addition to various nutrients, pesticides pour into streams and leach into groundwater supplies. In many agricultural regions, well water contains such a high concentration of nitrate that it is unfit to drink. To illustrate, we give below this case study from the United States of America.
The early European settlers found millions of hectares of virgin land in America. The eastern coast was heavily forested and so it was natural that they set their eyes on the West. It was the rich topsoil and reckless, treeless expanses, promising easy plowing, sowing and reaping, which attracted them. In 1889, the Oklahoma Territory was opened for homesteading. A few weeks later, the non-Indian population there rose from almost nil to close to 60,000. By 1900, the population was 390,000 people living off the wealth of the soil. However, over a period of 20 to 35 years, improper farming practices led to a decline in soil fertility.
Poor fertilization and loss of soil due to wind and water erosion took their toll. Finally, when a drought struck, the seeds failed to sprout and that resulted in a disaster. In 1934, a summer wind stripped entire country of their topsoil and even flew some of this dust more than 1500km eastward into the Atlantic Ocean. Altogether, 3.5Mha of farmland were destroyed, and productivity of an additional 30Mha was seriously reduced. Dangerous wind erosion continues to this day and some agronomists fear that another major Dust Bowl is in the making.