🧫 Characteristics and

This lesson explains key concepts in a structured way and connects them to practical agricultural applications and exam-oriented understanding.

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Characteristics and

Learning objective

To study the characteristics of heavy metal contaminated soils and remediation

Soil contamination

Soil contamination is the presence of man-made chemicals or other alteration of the natural

soil environment. This type of contamination typically arises from the rupture of underground

storage tanks, application of pesticides, percolation of contaminated surface water to subsurface

strata, leaching of wastes from landfills or direct discharge of industrial wastes to the soil. The most

common chemicals involved are petroleum hydrocarbons, solvents, pesticides, lead and other

heavy metals. This occurrence of this phenomenon is correlated with the degree of industrialization

and intensity of chemical usage. The concern over soil contamination stems primarily from health

risks, both of direct contact and from secondary contamination of water supplies.

The unscientific disposal of untreated or under-treated effluents has resulted in

accumulation of heavy metals in land and water bodies. Heavy metal contamination due to the

sewage and sludge application to soils imposes a major limitation on potential land use. Cultivated

areas under peri-urban agriculture are worst affected by this problem. The heavy metals

accumulating in soil may get entry into the human and animal food chain through the crops grown

on it.

Heavy metals prevailing in soils and their regulatory limits

Elements	Conc. range (mg kg-1)	Regulatory limit (mg kg-1)
Lead	1-6900	600
Cadmium	0.1-345	100
Arsenic	0.1-102	20
Chromium	0.005-3950	100

Mercury	0.01-1800	270
Copper	0.03-1550	600
Zinc	0.15-5000	1500

Management of soil pollution

Bioremediation

Bioremediation can be defined as any process that uses microorganisms, fungi, green

plants or their enzymes to return the environment altered by contaminants to its original condition.

Bioremediation may be employed to attack specific soil contaminants, such as degradation of

chlorinated hydrocarbons by bacteria. An example of a more general approach is the clanup of oil

spills by the addition of nitrate and / or sulfate fertilizers to facilitate the decomposition of crude oil

by indigenous or exogenous bacteria.

Important and widely reported hyper-accumulators used for metal remediation

Elements	Plant species	Max conc. (mg kg-1)
Cadmium	Thlaspi caerulescens	500
Copper	Ipomoea alpina	12300
Cobalt	Haumaniuastrum robertii	10200
Lead	Thlaspi rotundifolium, Brassica juncea, Zea mays	8200
Nickel	Alyssum lesbiacum, Sebertia accuminata	47500
Zinc	Thlaspi caerulescens, Brassica juncea, B. oleracea, B. campestris	51600
Selenium	Brassica juncea, B. napus	900
Chromium	Brassica juncea, Helianthus annus	1400

Microorganisms used for metal remediation

Elements	Microorganisms
Cadmium	Citrobacter spp.
Copper	Bacillus spp.
Cobalt	Zooglea spp.
Nickel	Zooglea spp.
Zinc	Bacillus spp.
Chromium	Pseudomonas ambigua, Chlamydomonas sp, Oscillatoria sp., Arthrobacter sp., Agrobacterium sp..

It is therefore imperative that to solve the soil chemical constraints and make the lands highly

productive on a sustainable basis, we need to develop the technologies suitable to specific

locations which will be economically feasible and workable at farmer’s field. So we have to give

emphasis on increasing the current yield level and at the same time develop suitable technologies

to reclaim the problem soils.

Questions to Ponder

1)What are heavy metals?

2. What are the long term effects of irrigation with distillery spent wash?

3)Which effluent contains high amounts of chromium?

4)What is the effect of heavy metals in foraging animals?

5)What are the critical levels of heavy metals in soils ?

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Summary Cheat Sheet

Key Recall Points

• Characteristics and is exam-relevant for SSAC122 and objective questions in soil science.
• Use soil-test based interpretation with focus on pH, CEC, and nutrient availability.
• Apply the 4R principle: right source, right rate, right time, and right method.

Exam Traps

• Do not mix up soil fertility concepts with fertilizer quantity alone.
• Numerical and term-based questions often test definitions, units, and threshold values.
• In problem-solving, interpretation must follow soil reaction, crop stage, and management context.

References

3 sources • [1] [2] [3]

[1]

ICAR e-Course: Soil Chemistry, Soil Fertility and Nutrient Management

Official

[2]

Brady and Weil, The Nature and Properties of Soils

Book

[3]

Indian Council of Agricultural Research

Official