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180 A. Branzini and M.S. Zubillaga

in soils, as it negatively influences the activity of microorganisms and earthworms,
thus retarding the breakdown of organic matter (Greany 2005). The Zn sources are
soil, zinc oxide, zinc sulfate, and zinc chelate, and Zn 2+ cation is the predominate
form taken up by plants.

10.1.3 Cadmium

Cadmium compounds are, compared to other heavy metals, relatively water solu-
ble. Therefore, these compounds are further mobile and available in soil and tend to
bioaccumulate. The average natural abundance of Cd in the earth’s crust has most
often been reported from 0.1 to 0.5 ppm. In contaminated soils, Cd is derived from
both natural and anthropogenic sources. Natural sources include underlying solid
rock or transported parent material such as glacial till and alluvium. Anthropogenic
input to soils occurs by aerial deposition and sewage sludge, manure, and phosphate
fertilizer application. The major factors governing Cd speciation, adsorption, and
distribution in soils are pH, soluble organic matter content, hydrous metal oxide
content, clay content and type, presence of organic and inorganic ligands, and
competition from other metal ions (Kim and Kim 2010). Its persistence in the
environment and its relatively rapid uptake and accumulation by food chain crops
are factors contributing to its potential environmental hazards. Cadmium
3
concentrations of air ambient rarely exceed 0.01 g m . However, cigarette smok-
ing adds considerably to Cd input via inhalation. On the other hand, even though
acute Cd toxicity caused by food consumption is rare, chronic exposure to high Cd
levels in food can significantly increase the accumulation of Cd in certain body
organs. Cd accumulates in the human body and especially in the kidneys. When Cd
concentration in human body reaches levels considered to be harmful
[>200 mg kg 1 wet weight in the kidney cortex according to Kjellstrom and
Nordberg (1978)], this metal could induce kidney damage and led to its dysfunction
with impaired reabsorption of, for instance, proteins, glucose, and amino acids.

10.1.4 Nickel

Nickel combined with other elements occurs naturally in the earth’s crust. It is
found in all soils, and is emitted from volcanoes. However, it normally occurs at
very low levels in the environment, and it is primarily found combined with oxygen
or sulfur as oxides or sulfides. Soil usually contains between 4 and 80 parts of nickel
in a million parts of soil (ppm). The highest soil concentrations (up to 9,000 ppm)
are found near industries that extract nickel from ore. Ni can also be released in
industrial wastewater. As a result, a lot of Ni released into the environment ends up
in soil or sediment where it strongly attaches to particles containing iron or
manganese. Under acidic conditions, Ni is more mobile in soil and might seep