Page 217 - Soil Degradation, Conservation and Remediation

206 6 Soil Pollution

Table 6.8 The permissible limits of some heavy metals in plant food
Permissible limit
−1
Metal Plant food material (mg kg ) References
Copper All food 10.00 CAC ( 1993 )
Cadmium All food 0.05 Walker ( 1988 )
Vegetables and fruits, excluding leafy 0.05 Graffham ( 2006 )
vegetables, fresh herbs, mushrooms
Leafy vegetables, fresh herbs, celeriac, 0.20 Graffham ( 2006 )
and all cultivated fungi
Stem vegetables, root vegetables, and potatoes 0.10 Graffham ( 2006 )
(peeled)
Lead Cereals and legumes 0.20 CAC ( 2003 )
Vegetables, excluding brassica, leafy 0.10 Graffham ( 2006 )
vegetables, fresh herbs, and all fungi
For potatoes the maximum level applies
to peeled potatoes
Brassica, leafy vegetables, and all 0.30 Graffham ( 2006 )
cultivated fungi
Zinc Grains 50.00 USDA ( 2003 )
Beans 100.00
WHO permissible limits in spices for Cu, Ni, Zn, Fe, Pb, and Hg are 50, 50, 100, 300, 100, and
−1
10 mg kg , respectively (Nkansan and Amoako 2010 )

mercury, and lead) have no biological role and are detrimental to the organisms even
at very low concentrations. High levels of both of the essential and nonessential
metals are toxic to the soil microorganisms (Pawloska and Charvat 2004 ). Heavy
metals adversely affect growth, morphology, and biochemical activities of microor-
ganisms and ultimately reduce their biomass and diversity (Roane and Pepper
2000 ). Heavy metals can damage the cell membranes, alter enzymes specifi city,
disrupt cellular functions, and damage the structure of the DNA. Toxicity of these
heavy metals occurs through the displacement of essential metals from their native
binding sites or through ligand interactions (Bruins et al. 2000 ). Also, toxicity can
occur as a result of alterations in the conformational structure of the nucleic acids
and proteins and interference with oxidative phosphorylation and osmotic balance
(Bruins et al. 2000 ). Heavy metals bring about change of the diversity, population
size, and overall activity of the soil microbial communities (Kelly et al. 2003 ). Leita
et al. ( 1995 ) studied influence of Pb, Cd, and Ti on microbial biomass, survival, and

activity during a laboratory incubation of soil. In comparison to uncontaminated
soil, the microbial biomass C decreased sharply in soil contaminated with Cd and

Ti, whereas the addition of Pb did not have any significant inhibitory effect on the
level of microbial biomass C. Long-term heavy metal contamination of soils reduces
microbial respiration (Doelman and Haanstra 1984 ). Many reports have shown
large reductions in microbial activity due to short-term exposure to toxic metals
too (Hemida et al. 1997 ). Bacterial activity, measured by thymidine incorporation
technique, had been shown to be very sensitive to metal pollution both under labo-

ratory and field conditions (Diaz-Ravina and Baath 1996 ). The size of mycorrhizal
roots has been found to decrease in soils containing high concentrations of heavy