A study showed that the heavy metals affect soil in two ways. One, they hinder the activities of micro-organisms that help recycle nutrients and aid in plant growth, and two, they disrupt the activities of enzymes. These findings are the results of a study carried out by researchers from the Department of Geology and Geophysics, IIT, Kharagpur. The effects of heavy metals on tiny aquatic plants in the Dhapa dumping site in Kolkata prompted the study.

Not very far from the dumping site, farmers grow vegetables on lands irrigated by sewage water. The findings of the study were published in Chemosphere (Vol 72, No 4).

For the study, the researchers took soil samples (0-20 cm depth) from six selected sites. The metal content was found to be higher in sewage-irrigated soil than in groundwater irrigated ones. “Such high levels of metals inhibited the growth of micro-organisms and their activities,” says Pradip Bhattacharyya, lead author of the study.
The study also found that heavy metals reduced the activity of some important soil enzymes like b-glucosidase, urease, phosphatase and sulphatase. The b-glucosidase is involved in mineralisation of soil organic matter and urease, phosphatase and sulphatase aid in the release of nitrogen, phosphorous, and sulphur from organic matter. Groundwater-irrigated soils showed the highest activity for all the investigated enzymes.

Metals were bound to carbonates and oxides as well as water soluble and exchangeable forms. “Of all the metals, cadmium and chromium are most harmful in crippling the soil enzymes,” says Bhattacharyya.
The study revealed that water-soluble and exchangeable metal forms were more harmful than those bound to carbonates and oxides. Such metal forms are toxic because they are easily released into water as ions. Metals ions bind to soil enzymes and inactivate them.


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