Agriculture Reference
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solution, contain soluble silicon substances. These are the products of mineral weathering or
dissolving.
Monosilicic acid possesses high chemical activity (Iler, 1979; Lindsay, 1979).
Monosilicic acid can react with aluminum, iron, and manganese with the formation of slightly
soluble silicates (Lumsdon and Farmer, 1985):
Al
2
Si
2
O
5
+ 2H
+
+ 3H
2
O = 2Al
3+
+ 2H
4
SiO
4
Al
2
Si
2
O
5
(OH)
4
+ 6H
+
= 2Al
3+
+ 2H
4
SiO
4
+ H
2
O
Fe
2
SiO
4
+ 4H
+
= 2Fe
2+
+ 2H
4
SiO
4
MnSiO
3
+ 2H
+
+ H
2
O = Mn
2+
+ 2H
4
SiO
4
Mn
2
SiO
4
+ 4H
+
= 2Mn
2+
+ H
4
SiO
4
Monosilicic acid under different concentrations is able to combine with heavy metals
(Cd, Pb, Zn, Hg, and others) forming soluble complex compounds if monosilicic acid
concentration is slight (Schindler et al., 1976) and slightly soluble heavy metal silicates when
the concentration of monosilicic acid is greater in the system (Lindsay, 1979).
ZnSiO
4
+ 4H
+
= 2Zn
2+
+ H
4
SiO
4
PbSiO
4
+ 4H
+
= 2Pb
2+
+ H
4
SiO
4
The anion of monosilicic acid [Si(OH)
3
]
-
can replace the phosphate anion [HPO
4
]
2-
from
calcium, magnesium, aluminum, and iron phosphates (Matichenkov, 2007).
In natural solutions, oligomers of silicic acid are present as well, they contain 2 and more
(up to 100) atoms of Si (Knight and Kinrade, 2001). Usually these substances are tested
together with monosilicic acid, but they possess some difference in chemistry, compared with
monosilicic acid (Matichenkov and Bocharnikova, 2001; Matichenkov et al., 2011).
However, the knowledge about this form of soluble Si is very low.
Polysilicic acids with high content of Si atoms (more than 100) are an integral component
of the natural solution as well. Unlike monosilicic acid, polysilicic acid is chemically inert
and basically acts as an adsorbent and forms colloidal particles (Jacinin, 1994).
n(Si(OH)
4
)
(SiO
2
) + 2n (H
2
O) or
[Si
n
O
2n-(nx/2)
(OH)
nx
] + mSi(OH)
4
= [Si
n + m
O
2n-(n2x/2 + 2m(2-p)
(OH)
nx + 4m - p
]
Polysilicic acids are readily sorbed by minerals and form siloxane bridges (Chadwick et
al., 1987). Since polysilicic acids are highly water saturated, they may have an effect on the
soil water-holding capacity. Polysilicic acids have been found to be important for formation
of soil structure (Matichenkov and Ammosova, 1996).
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