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SOIL POLLUTION AND PLANTS
Most of the atmospheric pollutants finally come down and settle on the ground surface, causing soil pollution. Pollutants also come back to soil when polluted water is used for irrigation. Many pollutants are directly deposited on the soil as sewage, industrial effluents and chemical fertilizers. The soil pollution by various pollutants has far reaching effects on plants and vegetation.
Effects of soil acidification
The oxides of sulphur and nitrogen, chlorides, fluorides, ammonium etc. emitted into the atmosphere in combustion from various industries come down as dry or wet deposition (acid rain) onto the soil and lower the soil pH. Increased acidity of soil results in following effects.
The activity of soil microbes, particularly of decomposers, is reduced. The decomposition of organic matter and consequently nutrient cycling in the soil is reduced. It ultimately adversely affects the growth of plants.
The bases in the soil are leached down due to soil acidity. As exchangeable bases become deficient in the soil, plant growth is reduced due to nutrient deficiency.
The roots, particularly the root hairs, are damages resulting in reduction in nutrient uptake by plants.
Increased acidity mobilizes heavy metals like Al, Cd, Zn, Hg, Mn, Fe etc. These spread rapidly in the soil along with soil water and reach concentrations toxic to plants. Consequently, plants show species specific metal toxicity symptoms. Al-toxicity generally damages root hairs and reduces nutrient uptake while Fe-toxicity has general adverse effect on plant growth.
In some soils, acidification increases weathering of silicate minerals destroying the mineral structure of the soil. This leads to poor growth of vegetation in general.
In some marginal soils and grasslands, acidification increases the supply of plant nutrients like sulphur and nitrogen. The vegetation is thus, benefited by soil acidification and plants may show better growth.
Effects of pesticides
Various pesticides, insecticides, fungicides etc are used in agriculture as foliar spray or are applied to soil far in excess to the requirement causing soil pollution. These substances pollute the soil depending upon their volatility, biodegradability, persistence, leaching, chemical reactivity and adsorption on the soil particles. Many of these substances form cations, are adsorbed on silicate clay micelle or humus molecules on the pH-dependent exchangeable charge sites and are later absorbed by the plants.
Absorbed pesticides substances produce characteristic species-specific toxicity symptoms in plants just like their aerial overdose.
Fungicides reduce abundance of soil fungi and actinomycetes and interfere with decomposition of soil organic matter adversely affecting the nutrient cycling.
Pesticides increase the abundance of some bacterial species, particularly of ammonifying bacteria while reduce the abundance of some susceptible bacteria.
Insecticides reduce the abundance of predator soil microbes and consequently increase the abundance of their prey species.
In general, species composition of the soil microflora and fauna is changed by pesticide substances .
The inorganic pesticides contain arsenic and sulphur. These substances cause toxicity symptoms like yellowing, necrosis, shot holes, premature defoliation etc. in plants.
Effects of herbicides
Various herbicides are used for weed control in agricultural practice. General effects of some common herbicide substances on the plants are as follows. The symptoms in response to a particular herbicide may be characteristic but their development depends upon the dose to which plants are exposed, rate of growth of plant, weather conditions and the plant species.
Acetanilides (e.g. Prochlor, Metachlor) cause stunting of plants and roots. In brassicas, these cause yellow, red and blue colouration of leaves with pronounced stunting.
Amides and carbamates (E.g. Diphenamide, Propyzamide, Chlorpropham and Asulam) cause stunting without chlorosis or leaf-scorch in mild doses. Crops generally become greener in colour. Roots show thickening and stunting. In the cereals, coleptile becomes stunted and swollen.
Benzonitriles (e.g. Dichlorobenil) cause stunting of plants and thickening of roots. In shrubs and bushes, bark at the ground level develops necrosis. In some cases, marginal yellowing occurs in the leaves.
Aliphatic acids (e.g. TCA, Dolpon) cause stunted growth of shoot, loss of leaf wax and mild irregular chlorosis.
Growth regulators (e.g. 2,4 D, MCPA, Chlopyralid, and Mecoprop) cause hormone-type distortion in leaves and fruits. In the brassicas, stem splitting occurs. In the cereals, ears become distorted, becoming blind or with shriveled grains.
Ureas and Uracils (e.g. Diuron, Linuron, Bromocil, Lencil) cause inhibition of photosynthesis leading to yellowing along the veins that later extends across whole leaf. In the cereals, chlorosis starts in the middle of leaf but extends quickly to the tip.
Sulphonyl ureas (e.g. Chlorsulfuron) cause inhibition of growth in susceptible plants and these ultimately die due to root growth being stopped.
Triazinones (e.g. Metribuzin) cause inhibition of photosynthesis that leads to yellowing and tip-scorch in the leaves.
Triazines (e.g. Atrazine, Simazine) cause chlorosis and necrosis spreading towards tips of leaves and inhibit photosynthesis. This is more severe in older leaves.
Thiocarbamates and ethofumesates cause chlorosis, stunting and leaf-curling. Younger leaves become stuck to older leaves. In the cereals, this condition often leads to characteristic pink colouration at the base of the stem.
Effects of sewage and ash pollution
Sewage matter is commonly used as fertilizer or deposited as waste on the soil. Effects of such pollution are mostly common to all plants.
The organic matter in sewage decomposes and produces nitrogenous substances that become excessive in the soil and harm the vegetation.
Decomposition of sewage may also release various toxic heavy metals that cause characteristic heavy metal toxicity symptoms in plants.
Detergent substances may also be released from sewage causing characteristic injury to plants.
Ash produced mainly from combustion of coal in thermal and industrial plants used for landfilling or deposited on soil makes the soil unfit for vegetation. It may release many toxic substances in the soil causing characteristic plant injuries.
Effects of chemical fertilizers
Chemical fertilizers are generally used far in excess of the requirements of the crop. The unutilized fertilizers cause soil pollution.
Toxic concentrations of nitrogen fertilizers cause characteristic symptoms of nitrite or nitrate toxicity in plants, particularly in the leaves. Nitrogenous fertilizers generally cause
deficiency of potassium,
increased carbohydrate storage and reduced proteins,
alteration in amino acid balance and consequently change in the quality of proteins.
Ammonium fertilizers produce ammonia around the roots that may escape the soil and cause ammonia injury to plants.
Ammonium and nitrate produce acids in the soil and increase soil acidity.
Nitrate and nitrite bacteria are reduced while ammonifying bacteria are increased in the soil disturbing the nitrogen cycle.
Excessive potash in the soil decreases ascorbic acid and carotene in the plants.
Superphosphates cause deficiency of Cu and Zn in plants by interfering with their uptake.
Excessive lime prevents the release of Co, Ni, Mn and Zn from the soil and their uptake by plants is reduced causing their deficiency symptoms.
Excessive deposition of various substances released from chemical fertilizers into the soil generally causes their over-absorption by plants. These over-absorbed substances become accumulated in plant parts (bioaccumulation) e.g. nitrogen and sulphur are deposited in the leaves.
Effects of industrial effluents
Various inorganic and organic substances are present in the industrial effluents. These substances mostly remain tied up in the soil and are not readily available to plants. However, they affect various soil characteristics.
Effluents affect the mineral structure, soil pH, exchangeable base status etc. of the soil and thus indirectly affect the plants
The pH of the soil is disturbed making soil either acidic or alkaline.
Various inorganic and organic chemicals are accumulated in the soil up to levels toxic to plants.
In highly polluted soils, plants absorb and accumulate toxic substances (bioaccumulation). These substances may or may not produce direct injury symptoms in plants but are passed on to higher trophic levels (biomagnification).
Effects of radioactive pollutants
A variety of radioactive waste materials like Strontium-90, Cesium-137, Iodine-131, Plutonium, Uranium, Americium, Curium, Neptunium etc. are added to soil from nuclear activities. These substances may be washed into water or may be directly added to water that is used as coolant in nuclear power plants.
Uptake of radioactive substances by plants depends upon pH and organic matter status of the soil.
Various radioactive materials show different solubility and absorption by plants. Plant uptake is generally highest for Neptunium, lowest for Plutonium and intermediate for Americium and Curium.
Strontium-90 behaves as Ca in the soil and is absorbed by plants like it.
Cesium-137 behaves like K but its uptake by plants is very limited.
Absorbed radioactive substances are generally accumulated in the vegetative parts of the plants, particularly the leaves. Their accumulation in crop fruits and seeds is very low.
Accumulation of absorbed radioactive substances in the plants may be up to 100 times their concentration in the soil of water.
Radiation from radioactive substances may also adversely affect the plants. Pines are eliminated on exposure to 20-30 roentegen/day while most plants die at 200 roentgen/day. Only lichens and mosses are highly resistant to radiation.
Radiation also damages chromosomes. It increases the frequency of chromosomal aberrations and causes genetic mutations. Such genetic changes may adversely affect plant metabolism or change their characteristics in subsequent generations.