Soil Pollution
Introduction
Soil Pollution is a considered as a major challenge for healthy environment. The weathering of earth’s crust forms, soil over the centuries that supports the variety of microscopic and macroscopic life-forms.
The top most layer of soil is composed of minerals of various sizes and organic matters along with pores filled with air and water.
It is chiefly the lack of human awareness that makes land lose its fertility simultaneously increasing its alkalescency and acidity. As a result, the surface of the soil erodes. This erosion is called soil pollution.
Causes and Types of Soil pollution
Soil gets polluted by a number of ways. The major kinds of soil pollution are
Acidification.
Salinization.
Agrochemical pollution.
Contamination by metalliferous waste.
1. Acidification:
Acidification has a number of natural and anthropogenic causes. The main natural causes are long-term leaching and microbial respiration. The acids found in rainwater (carbonic acid) and in decomposing organic material (humic and fulvic acids) can stimulate leaching by dissociating into H+ ions and their component anions which then displace or attract base cations from the soil exchange complex.
The main anthropogenic causes of acidification include certain land use practices, such as needle-leaf afforestation, excessive use of inorganic nitrogen fertilizers, land drainage and acid deposition resulting from urban and industrial pollution. Needle-leaf afforestation has been associated with the acidification of soils and surface waters for a number of reasons. First, needle-leaf trees produce litter which is very acidic in comparison with most broadleaf species. Second, because of their high canopy surface area, needle-leaf trees are able to ‘scavenge’ acid pollutants from the atmosphere, later releasing them into the soil via through fall and stem flow.
Excessive use of inorganic nitrogen fertilizers in agricultural systems has also been associated with soil acidification, partly through the process of nitrification.
Acidification of soils and associated nutrient leaching also cause damage to the trees in forested areas.
2. Salinization:
Although salinization occurs naturally in semi-arid and arid environments, it is often exacerbated as a result of human activity. In parts of southwest Australia, for example, removal of indigenous eucalyptus forest has resulted in extensive salinization of soils. This has occurred because the deeply-rooted trees have been replaced by shallow-rooted grasses and crops, which are less effective in lowering the ground-water level. Capillary action is most intense, and salinity is greatest in soils where the water table is within about 2 meter of the surface. Another important cause of soil salinity is poor irrigation practice. Over-watering leads to a rise in the water table which, in turn, causes enhanced capillary action. Similarly, poor maintenance of irrigation channels and canals results in leakage of water into adjacent agricultural land. This has contributed to increased soil salinity in parts of the Indus Valley in Pakistan.
3. Agrochemical pollution
In recent decades, the use of inorganic fertilizers has increased dramatically at the expense of more traditional organic nutrient treatments. Between 1952 and 1985, the global use of fertilizers increased from 14 million tons to 125 million tons, an increase of almost 900 percent. Inorganic fertilizers are used in preference to organic treatments because the nutrients are in a more readily available form and are released rapidly after applications. Organic material releases its nutrients slowly, through decomposition processes, and only when conditions are suitable (warm and moist), not necessarily when crops need them.
Fertilizers are applied in a variety of forms — solution, suspension, emulsion and solid. The solid forms vary in particle size from fine powder to coarse granules and either spread evenly (broadcast) over the soil surface or mechanically placed, by drilling, into the rhizosphere. Generally the rate of nutrient release decreases with increasing particle size. Fertilizers are based on compounds of plant macro-nutrients (e.g. nitrogen, phosphorus and potassium) and micro-nutrients (e.g., Zinc, copper, boron and molybdenum). A variety of nutrient combinations are available depending on the nature of the nutrient problem. Increased use of inorganic fertilizers eventually decreases the soil fertility at a faster rate. It also results in the increase of harmful insect and pest attacks.
4. Urban and Industrial Pollution
Urban and industrial development has been associated with both physical degradation and chemical contamination of soils. Problems of physical degradation include erosion, compaction and structural damage resulting from construction activities and opencast mineral extraction. Similarly, chemical problems result from waste disposal activities, discharge and spillage of liquid effluents and atmospheric emission including acid deposition.