Page 216 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 205

6.2.12.4 Heavy Metals in Plant Food
The levels of heavy metals in edible plant parts collected from crop fi elds and
markets of vegetables, cereals, fruits, and spices have considerably been investigated.
Metal contamination of plant materials have been reported from metal- contaminated
places such as mining and industrial areas and in crop fields amended with sludge-

based fertilizers or irrigated with contaminated water.
Lead concentration in polished rice from paddy grown in a contaminated site
− 1
of China reached 0.69 mg kg , which is higher than the maximum allowable
− 1
concentration by the safety criteria for milled rice for China (0.20 mg kg ).
Cadmium contents in 31 % of the rice samples exceeded the national maximum
allowable concentration (Fu et al. 2008 ). Levels of Cd, Zn, Pb, and Cr in roots, stems,
and leaves of the vegetable plants Amaranthus viridis and Talinum triangulare
planted on poultry dumpsite of Nigeria were 0.62–2.74, 50.67–102.98, 2.27–7.21, and
− 1
0.64–4.45 mg kg , respectively. Some of these values were above the safe levels

(Adefila et al. 2010 ). Metal levels in vegetables, including parsley, onion, lettuce,
garlic, peppermint, spinach, broad bean, chard, purslane, grapevine leaves, and
fruits, including tomato, cherry, grape, and strawberry grown in Manisa region of Turkey,
ranged from 0.56 to 329.7, 0.01 to 5.67, 0.26 to 30.68, 0.001 to 0.97, and 0 to 0.06 mg kg −
1 for Fe, Cu, Zn, Pb, and Cd, respectively (Bagdatlioglu et al. 2010 ). Contamination
with heavy metals of fruits including orange, mango, almond, lemon, sweet orange,
grapefruits, chiku, papaya, muskmelon, apple, quince, grape, strawberry, banana,
pineapple, papaya, carambola, longan, wampee, date palm, and apricot have been
reported (Davarynejad et al. 2010 ). Concentrations of Cu, Ni, Zn, Fe, Pb, and Hg
of 15 common spices available at local markets in the Kumasi Metropolis of Ghana
− 1
ranged from 9 to 21, 31 to 50, 59 to 74, 110 to 494, 96 to 115, and 0.001 to 0.025 mg kg ,
respectively (Nkansan and Amoako 2010 ). Permissible limits of heavy metals in
edible parts of plants are shown in Table 6.8 .

6.2.12.5 Toxicity of Heavy Metals

Organisms are exposed to soil contaminants by direct contact, handling, absorption,
or ingestion and food intake. Heavy metals are bioaccumulated by incorporating
into tissues, and as they are transferred through the food chain, they are biomagnifi ed
at higher trophic levels.

Effect of Heavy Metals on Soil Microorganisms

Some heavy metals are essential micronutrients (cobalt, chromium, nickel, iron,
manganese, and zinc) for soil microorganisms (Bruins et al. 2000 ). They are
involved in redox processes, in order to stabilize molecules through electrostatic
interactions, as catalysts in enzymatic reactions, and regulating the osmotic balance
(Hussein et al. 2005 ). On the other hand, some other heavy metals (cadmium,