Arsenic Toxicity and Tolerance Mechanisms in Plants: An Overview - Crop Improvement: New Approaches and Modern Techniques - page 365

PhysicalCharacteristicsofArsenic
• Arsenic is tin white tarnishes to dark grey or black in colour.
• It is metallic but tarnishes to dull in luster.
• Its crystals are opaque.
• Crystals are rare pseudocubic rhombohediral and acircular radial aggregates.
It is usually found in fine-grained masses with concentric bands or botryoidal
crusts. Allied minerals of arsenic are barite, cinnabar and neckline. Others are
poisonous.
• Arsenic is found to occur in France, Kangsberg, Norway, Somany and Harz
Mountains, Germany, Honshu, Japah, England, Italy, and Santa Cruz, Arizona
and New Jersey, USA.
• Best field indicators are tarnish, density, softness, crystal habits, color, garlic,
small and association.
BehaviourofArsenic
Both arsenic and phosphorus have similar chemical properties, therefore they act
similarly in soil. Phosphorous and arsenic competes for soil fixation sites and for
plant uptake (Adriano 1986 ). By decreasing soil phosphorus level the phytotoxicity
of arsenic increases (Rumberg et al. 1960 ). Other experiments showed that addi-
tional phosphorus may increase arsenic phytotoxicity by releasing more arsenic into
solution (Jacobs and Keeney 1970 ).
AvailabilityofArsenic
Arsenic availability cannot be determined by the total arsenic concentration in soils
(Adriano 1986 ). Even though a limited quantity of arsenic in soil is readily mobile
and the rest is not available to plants because it is associated with iron (Fe) and
almunium (Al).
In soil the solubility of reduced form of arsenic (arsenite) is more oxidized form
(arsenate). Soluble arsenic concentration is directly proportional to the plant arsenic
toxicity, (Kabata-Pendias and Pendias 2001 ; Sturchio et al. 2011 ).
Many soil factors like soil pH affect the arsenic availability (Adriano 2001 ).
Therefore, soil pH is important because it plays a vital role in arsenic speciation and
leachability. The optimum adsorption for arsenite is approximately at pH 7.0 while
as arsenate adsorbs at pH 4.0 (Pierce and Moore 1982 ). On the whole, low soil pH,
clays makes the amorphous silicates and metal oxides protonated and is then able to
absorb arsenic anions present in the soil. Arsenic is less mobile at low pH as most
arsenic is present as arsenate in soils and at this pH there are high concentrations of