Indicators of Soil Pollution

Plants can be used to indicate the nature of the soil (whether acidic, basic or saline) and other pollutions. Besides plants micro organisms are the rapid detectors of environmental pollution.

9.8.1 Plants as Indicators of Soil Pollution

Plants require a number of macro-nutrients and micro- nutrients (see sec. 8.6). Besides these, there are other elements which are essential for plant growth like sodium for Chenopodiaceae, cobalt for Fabaceae, aluminium for fems, silicon for diatoms and gold for Equisetum. Certain plants can withstand the presence of heavy metals in higher concentration, e.g., silene plants to zinc, liver worts and mosses to copper, serpentine to chromium and nickel, Fe, Stucca species to lead and Acidophilic grasses to aluminium.

Some plants are called soil indicator plants since they reveal the quality of substrate (on which they grow) in the soil. (F.E. Clements, 1920, Ecologist Division of Plant Biology, Camegie, Institute of Washington, USA).

Plants belonging to the species of sphagnum can grow in acidic soil (pH

3.5 - 8.5) e.g., bog soils. Such plants are considered as strongly acidophilic and are sensitive to alkaline soils. Certain grasses like Deschampsiaflexuosa, Calluna vulgaris, Sarothamus scoparis are also indicators of acidic soil; however, these can grow in neutral soil and can tolerate weakly alkaline soils.

Alkaline soils have high concentration of Na and K ions. Strongly alkaline soils are injurious to glycophytes. Bacteria of the Basiphilic species do not survive if the pH of soil falls below 6.0. Tussilagefeifare is not affected in the pH range 7.0 - 8.5; it can also grow in neutral soil and can tolerate pH 4. It is known as Basiphilic acido - tolerant species.

Saline soils (pH below 8.5) have accumulated salts and interfere with the growth of non - halophytes.

Liverworts and some species of mosses are called copper mosses since they are associated with copper bearing substrates. These have been used for locating copper deposits. Copper mosses give information about metallic content in soil to geologists.

Plants provide an excellent means for indicating soil pollution, since they develop toxicity symptoms for a particular pollutant. Certain plant species like wolffia, chara, Utricularia grow well in polluted waters, while Anthoxanthum, Agrotis, Festuca and Impatiens are used as metallic tolerana of zinc, copper, lead and cadmium, respectively. Lichens and mosses are also used to indicate pollution. Leaf cabbage indicates accumulation of polycyclic hydrocarbons in the soil. Tolerant plant species are good indicators to monitor the incidence of particular soil pollution. Certain plant species besides acting as bio - indicators are also used as pollutant scavengers.

9.8.2 Organisms as Bio-Indicators

Some species of organisms are used as bio-indicators. Such species are referred to as bio-sensors or bio-accumulators. These can concentrate toxic metals, pesticides and organic industrial components in their tissues like gills, flesh, muscles, liver, bones, etc. (Also see Section 6.4.2.9).

9.8.3 Micro-Organisms as Bio-Indicators

Environmental pollution in soil as well as in water can be conveniently detected by some micro - organisms. Some organisms are sensitive to foreign materials and some others help in the bio decomposition of pollutants. The presence of toxic agents is indicated by the elimination of sensitive species. In petroleum belts and sediments, detection of polar lipids of archeobacteria is a convenient monitoring system. For soil pesticides, cyanobacteria is effectively used as bio - indicator. As an example, Nostoc microscopicum, Haplosiphon Welwitschii and H. Confervaceus indicate pesticide pollution of Dithane, BHC, Aldrex, Deltan, Roger and Phorate, etc. in the soil. Oil spillage can be monitored by Actinomycetes, bacteria, yeast and some filamentous fungi. In fact, waste oil can be absorbed by some species like Humicola, Verticillium, Scolecobasidium and Mortierella.