Soil erosion, demunition of forest and desertification result from land-clearance and over-harvesting. State and explain the undesirable effects of certain agricultural practices in the wise use of our natural resources.
Soil erosion is one form of soil degradation by which the topsoil is removed naturally by the wearing actions of rain and wind. Soil erosion may be a slow process and occur at an unnoticed rate. It may also occur at an alarming rate and cause serious loss of topsoil every year. Soil erosion is one of the major environmental problems faced by people today. It reduces agricultural and forestry production significantly and also degrades the quality of aquatic ecosystems.
In the farmland, soil erosion is caused by inappropriate practices such as intensive ploughing and over-grazing. Intensive ploughing, especially with the use of inappropriate machinery, can damage soil structure. The soil is loosened and large aggregates of soil particles are broken down. A large amount of powdery materials is formed. They form mud when mixed with rain water and seals the soil surface. So rainwater cannot be absorbed by the soil easily. This leads to increased runoff and soil loss. Ploughing also provides extra air and warmth. This speeds up the rate of bacterial deterioration. Humus, which helps to hold water, is lost at a faster rate, and this further speeds up soil erosion. Over-grazing leads to soil erosion. When herbivores are reared on a small piece of grassland, the rate of consumption of grass is usually faster than the rate of recovery. Eventually, the grass becomes too short or dies off. The process of erosion is speeded up as the soil is exposed. In addition, the trampling of the grassland by large populations of animals will make the soil compact. Rainwater cannot be easily absorbed by the soil. As a result, the soil becomes dry and loose, and soil erosion is speeded up.
Soil erosion causes soil loss, leading to the reduction of crop productivity and water pollution problems. The ecological impact of soil erosion by inappropriate practices can be summarized into two major areas: destruction of agricultural farmland and water pollution.
Soil erosion severely affects the stability and texture of soil. When the topsoil is removed, vital plant nutrients such as nitrogen, phosphorus and potassium are lost. The fertility of the soil decreases, leading to a reduced crop production. The breakdown of aggregates of soil particles due to intensive ploughing lowers the water-holding capacity of the soil, making it more susceptible to drought. If soil erosion is severe, the entire layer of topsoil and vegetation may be washed away. No more crops can be cultivated and the farmland may be abandoned.
Soil is washed away by heavy rainfall into streams and water courses below the eroded farmland. It can clog drainage ditches and stream channels, and may form silt in reservoirs and harbors. Pesticides and other agricultural chemicals are carried into aquatic bodies such as streams, rivers and lakes. They may contaminate the fish spawning areas and threaten aquatic life. Also, severe erosion and water runoff on mountain in slopes affect agricultural lands in the valley below. There may be a large quantity of sediment and flooding, which further decrease agricultural productivity and speed up water pollution problems.
Desertification is a process whereby productive farmlands in arid, semi-arid and dry sub-humid areas are degraded into desert.
Desertification is caused by two main categories of factors: natural factors and, more importantly, human activities.
The natural factors causing desertification include the following:
~The climate has a major influence through rainfall, solar radiation and wind, which affect the rates of physical and mechanical erosion as well as chemical and biological degradation of soil.
~The relief of a land affects the rate of soil erosion by water.
~The textile, structure, and chemical and biological status of soil are predominant factors determining the soil properties in dry sub-humid zones.
Human activities play a crucial role in the vulnerability of land to desertification. The reasons behind these activities are the increasing demand for food due to the rapid population growth, and inappropriate agricultural practices. The following are some of the human activities that cause desertification:
1. Uncontrolled use of fire for regenerating pasture, for hunting or for agricultural clearing
2. Over-grazing of selective vegetation
3. Removal of hedges, which can act as wind breaks, on soil so that the soil is more susceptible to erosion by wind
4. Over-harvesting which results in abandoned fields
5. Agricultural practices that result in continuous removal of soil nutrients
6. Monoculture of cash crops, leading to severe reduction in soil fertility
7. Inappropriate irrigation of soil, leading to salinization, waterlogging and abandoned fields eventually.
Farmlands are important natural resources. Humans rely on them for food. Desertification results in the loss of farmlands, and significantly threatens the living standard and well-being of people inhabiting the areas concerned. This also leads to social problems such as environmental refugees whose lands are too eroded for cultivation or rearing livestock.
Desertification also has serious impact on the natural environment. It breaks down the fragile balance that allows plant and animal life to develop in arid, semi-arid and dry sub-humid zones. This breakdown of the equilibrium represents the start of a process that destroys the natural and stable ecosystem.
Another problem is that the results of desertification in turn speed up the natural process of desertification. The vulnerability of soil wind and water erosion, the lowering of the water table, the impairment of the natural regeneration of vegetation, and the chemical degeneration of soils are all intensifying desertification.
Deforestation refers to the clearance of forests and the conversion of them for non-forest uses, including agriculture, animal grazing, timbering, and urbanization. Forests may also be destroyed by natural means such as forest fires. The forests can often recover gradually after forest fires. However, deforestation sue to human activities tends to be permanent.
Among the different purposes of deforestation, agriculture and animal grazing account for the major destruction of forest,. It is estimated that over half of the original forest cover on the earth has been cleared for these purposes.
Forests play a vital role in the biosphere. They act as a climatic buffer by retaining moisture in the air, cooling down the air, and maintaining the balance of carbon dioxide level in the atmosphere. They lock up nutrients and maintain their availability to other organisms. They hold the soil and retain rainwater. These help to prevent soil erosion, flooding, sedimentation and water pollution. Forests are also important as the habitats for a large variety of organisms.
The lands occupy only about 30% of earth’s surface. Only about 10%of the land surfaces are suitable for agricultural practices. It is estimated that the formation of fertile agricultural soil requires 3000years, but humans have already destroyed over 30% of their farmlands. This is mainly due to misuse of agricultural farmlands and inappropriate agricultural practices such as over-harvesting, monoculture and inappropriate irrigation techniques.
Cultivated fields are relatively unstable ecosystems because all vegetations except the cultivated crop plants are removed. This has already a great impact on the soil quality. Over-grazing further increases the impact on soil. When crops are harvested, plant tissued together with large quantities of minerals is taken away. Thus the nutrients required by the crops are rapidly depleted from the soil. If fallow periods are not sufficient and fertilizers are not applied appropriately, the soil will eventually become infertile.
Monoculture refers to an agricultural method by which the same crop is grown over deveral seasons on the same field. Today, agricultural production is mainly based on demands in the market rather than individual needs. Therefore, the monoculture of a profitable crop becomes a general practices of farmers.
Monoculture usually results in depletion of particular minerals in the soil. It also causes a lot of ecological problems. In fact many pests have very specific requirements for food and they attack particular types of plants. When one plant is attacked, the whole field can be affected. Diseases, which are also specific to individual types of crops, can also spread quickly. As a result, farmers rely heavily on chemicals to control pests. These chemicals will accumulate in soil, and will cause water pollution when they are washed away by heavy rainfall into streams and rivers.
Irrigated lands can give crop yields that are double or triple those with only natural watering by rain. Irrigation water is usually a dilute solution of various salts. The water is not totally absorbed by the soil. Part of it evaporates and leaves behind a thin crust of dissolved salts in the topsoil. The accumulation of salts is called salinization. Salinization usually causes stunt crop growth and lower yields. They may eventually kill plants and ruin the soil. In addition, farmers often apply large amounts of irrigation water on their farmlands. This will accumulate underground, envelops the deep roots of the crops and lowers their productivity. Inappropriate irrigation techniques will eventually cause the farmland not suitable for crop growth and becomes abandoned.
Soil erosion, demunition of forest and desertification result from land-clearance. Forest may be cleared for urban and industrial development. New urban and industrial areas are also converted from agricultural lands. This may have dramatics effects on the soil condition. Soil may be washed away from the bare ground during heavy rainfall. This may pollute the aquatic environment and also threaten aquatic lives.