2.2
Silicon: Occurrence and Sources
In accordance with multiple sources, the soils inherit their element composition
primarily from parent rocks that were subjected to geochemical as well as
pedochemical weathering processes primarily. As per the data evaluated, silicon is
the second most abundant element in the earth’s crust in terms of quantity after
oxygen, i.e. 27.7%. (Mitra 2015; Malhotra and Kapoor 2019). In the earth’s crust, Si
has been deposited in the form of quartz (SiO 2 ), sand, and sandstone (Rédei 2008;
Malhotra and Kapoor 2019). Within the soil, it comprises even up to 70% of soil
mass in the form of monosilicic acid, polysilicic acid as well as complexes with
organic and inorganic compounds such as aluminum oxides and hydroxides (Rao
and Susmitha 2017). Out of all, the most important form is plant-available form,
i.e. silicic acid (H 4 SiO 4 ), a non-charged plant-available molecule that considerably
ranges between 10 ppm to over 100 ppm (Epstein 2009; Liang et al. 2015; Zargar
et al. 2019). During the crop use, the polysilicic acid, and inorganic and organic
complexes act as important sinks/sources that replenish the monosilicic acid (Rao
and Susmitha 2017). In soil solution, the concentration of Si is equivalent even up to
many macroelements such as potassium (K), calcium (Ca), and phosphorus
(P) (Epstein 1994; Malhotra and Kapoor 2019). Silicon solubility in the soil is
affected by a variety of factors, which include irrigation water, the particle size of
the silicon fertilizer employed, critical soil characteristics (moisture, temperature,
and pH), presence of organic complexes concentration of iron (Fe), phosphate (P),
and aluminum (Al) ions as well as dissolution reactions occurring (Gérard et al.
2002; Tavakkoli et al. 2011; Rao and Susmitha 2017; Zargar et al. 2019). As per one
report, the range of silicon present in the soil around the globe lies within 50–400 g
silicon/kg of soil (Matichenkov and Calvert 2002; Haynes 2019). The silicon present
within soil improves water absorption capacity, soil physical and chemical
properties as well as maintain other nutrients in plant-available form by creating
silica bridges (Rao and Susmitha 2017; Malhotra and Kapoor 2019; Zargar et al.
2019). Compared to the soil, silicon comprises 0.0001% and 0.026% in terms of
quantity in oceans and humans, respectively (http://www.elementalmatter.info/ele
ment-silicon.htm).
On the other hand, the availability of Si in plants is low but ranges from 0.1 to
10% of dry weight (Hodson et al. 2005; Balakhnina and Borkowska 2013; Malhotra
and Kapoor 2019). In here, the most important fact to be noted is that concentration
of Si ranges distinctly within and among plant species which depend directly on the
capability of the roots to uptake Si (Hodson et al. 2005; Ma and Yamaji 2006;
Malhotra and Kapoor 2019). Plants have been categorized into three major classes
such as accumulators, intermediate, and excluders based on the Si accumulation in
their tissues (Mitani and Ma 2005; Marschner 2012; Luyckx et al. 2017). The best
examples of accumulators are members of Equisetales, Cyperales, and Poales
(especially rice, sugarcane, and maize) with a value of more than 1 for silicon/
calcium ratio. On the contrary, tomato and soybean are examples of Si excluders that
show the value of <0.5 for silicon/calcium. However, nettle and snapdragon are
examples of the intermediate type (Mitani and Ma 2005; Luyckx et al. 2017;
20
S. Mehta et al.
Silicon: Occurrence and Sources
In accordance with multiple sources, the soils inherit their element composition
primarily from parent rocks that were subjected to geochemical as well as
pedochemical weathering processes primarily. As per the data evaluated, silicon is
the second most abundant element in the earth’s crust in terms of quantity after
oxygen, i.e. 27.7%. (Mitra 2015; Malhotra and Kapoor 2019). In the earth’s crust, Si
has been deposited in the form of quartz (SiO 2 ), sand, and sandstone (Rédei 2008;
Malhotra and Kapoor 2019). Within the soil, it comprises even up to 70% of soil
mass in the form of monosilicic acid, polysilicic acid as well as complexes with
organic and inorganic compounds such as aluminum oxides and hydroxides (Rao
and Susmitha 2017). Out of all, the most important form is plant-available form,
i.e. silicic acid (H 4 SiO 4 ), a non-charged plant-available molecule that considerably
ranges between 10 ppm to over 100 ppm (Epstein 2009; Liang et al. 2015; Zargar
et al. 2019). During the crop use, the polysilicic acid, and inorganic and organic
complexes act as important sinks/sources that replenish the monosilicic acid (Rao
and Susmitha 2017). In soil solution, the concentration of Si is equivalent even up to
many macroelements such as potassium (K), calcium (Ca), and phosphorus
(P) (Epstein 1994; Malhotra and Kapoor 2019). Silicon solubility in the soil is
affected by a variety of factors, which include irrigation water, the particle size of
the silicon fertilizer employed, critical soil characteristics (moisture, temperature,
and pH), presence of organic complexes concentration of iron (Fe), phosphate (P),
and aluminum (Al) ions as well as dissolution reactions occurring (Gérard et al.
2002; Tavakkoli et al. 2011; Rao and Susmitha 2017; Zargar et al. 2019). As per one
report, the range of silicon present in the soil around the globe lies within 50–400 g
silicon/kg of soil (Matichenkov and Calvert 2002; Haynes 2019). The silicon present
within soil improves water absorption capacity, soil physical and chemical
properties as well as maintain other nutrients in plant-available form by creating
silica bridges (Rao and Susmitha 2017; Malhotra and Kapoor 2019; Zargar et al.
2019). Compared to the soil, silicon comprises 0.0001% and 0.026% in terms of
quantity in oceans and humans, respectively (http://www.elementalmatter.info/ele
ment-silicon.htm).
On the other hand, the availability of Si in plants is low but ranges from 0.1 to
10% of dry weight (Hodson et al. 2005; Balakhnina and Borkowska 2013; Malhotra
and Kapoor 2019). In here, the most important fact to be noted is that concentration
of Si ranges distinctly within and among plant species which depend directly on the
capability of the roots to uptake Si (Hodson et al. 2005; Ma and Yamaji 2006;
Malhotra and Kapoor 2019). Plants have been categorized into three major classes
such as accumulators, intermediate, and excluders based on the Si accumulation in
their tissues (Mitani and Ma 2005; Marschner 2012; Luyckx et al. 2017). The best
examples of accumulators are members of Equisetales, Cyperales, and Poales
(especially rice, sugarcane, and maize) with a value of more than 1 for silicon/
calcium ratio. On the contrary, tomato and soybean are examples of Si excluders that
show the value of <0.5 for silicon/calcium. However, nettle and snapdragon are
examples of the intermediate type (Mitani and Ma 2005; Luyckx et al. 2017;
20
S. Mehta et al.