Zinc [Zn, 30]
357
and Sadurski 2004). Complex Zn compounds are also likely to be formed in soil solution: ZnFe 2 O 4 , Zn(PO 4 ) 2 ·4H 2 O, Zn(PO 4 ) 2 , and ZnCO 3 (Barsova and Mozutova 2012).
Depending on the techniques used for obtaining soil solution, Zn concentrations
vary between about 20 and 570  μg/L. In very acidic soils (pH  < 4), Zn concentrations average 7,137 μg/L, whereas in contaminated soils its contents may reach
17,000 mg/L (Kabata-Pendias 2011).
Although Zn is very mobile in most soils, clay fractions, SOM, and P compounds are
capable of holding Zn quite strongly, especially in neutral and alkaline soils (Kumpiene
et al. 2008). SOM is known to be capable of bonding Zn in relatively stable forms, therefore its accumulation in organic soil horizons and in some peat is observed. However,
the stability of Zn–organic matter (OM) compounds are relatively low. The addition of
sewages sludge to soil increases the contents of two Zn species, mobile and exchangeable. In general, organic complexes of Zn increase its uptake by roots. Carbonates added
to contaminated chermozems increase Zn forms bound to Ca compounds and significantly decrease its amounts adsorbed on Fe and Mn hydroxides (Minkina et al. 2010).
Oxides and hydroxides of Al, Fe, and Mn also play important functions in Zn binding, in some soils. Especially goethite, αFeOOH, reveals a great affinity for Zn sorption.
Microorganisms may partly reduce this process, due to the accumulation of Zn in living
cells (Perelomov and Kandeler 2006). Also dissolved OM is an important factor affecting
Zn mobility in soils, in particular, soils with alkaline pH range of 7–7.5 (Wong et al. vide
Kabata-Pendias 2011). At higher pH values, Zn organic complexes may also account for
its solubility, whereas in acidic (mainly sandy) soils SOM seems to be the most important
soil component in binding this metal. The most important factors in controlling Zn mobility in most soils are clay minerals, hydrous oxides, and pH. However, in some soils, especially with high OM contents (e.g., peaty soils), Zn organic species are highly responsible
for its behavior. Apparently, there are two different mechanisms of Zn sorption: in acidic
media it is associated with cation exchange sites, whereas in alkaline media it is mainly
chemisorption, highly influenced by organic ligands.
In acidic sandy soils, Zn is always very mobile and is often easily leached down the
soil profiles. The immobilization of Zn occurs in soils rich in Ca, P, and S compounds, as
well as in clay minerals, especially allophane, imogolite, and montmorillonite. Zinc deficiencies, especially its phytoavailable species, result, most often, from management practices used during crop production (e.g., overliming, P fertilization, and OM amendment).
Zinc contamination of soils is relatively common. Anthropogenic Zn sources are
related to several industrial processes, and agricultural practices. Soils around nonferric smelters contain Zn up to about 1115 mg/kg (ATSDR 2005d). Soils surrounding
the Zn smelter, in Poland, contain this metal within the range of 202–4832 mg/kg
(Diatta et al. vide Kabata-Pendias 2011). Along high traffic roads, Zn content may be
up to about 900 mg/kg (Niesiobędzka 2012).
Agricultural Zn sources are mainly from various fertilizers, which contain its
variable amounts, as follows (in m/kg): triple superphosphate, 142–624; NPK fertilizers, 60–200; cow slurry, 580; pig slurry, 920; poultry slurry, 495; and sewage
sludge (of EU countries), 131–1670 (Kabata-Pendias 2011).
In soils heavily contaminated with Zn, the formation of Zn pyromorphite
[Zn 5 (PO 4 ) 3 OH] is observed mainly at the surface of grass roots, which suggests the
impact of the rhizosphere on this process. Some soil microorganisms (Rhizopus arrhizus)