Silicon (Si) is the second most abundant element in the Earth’s crust, and is an important component of mineral soils. Crystalline forms of Si are sparingly soluble and amorphous forms are somewhat more so. The main soluble form of Si in soils is monosilicic acid (H4SiO4) which is the form taken up by plant roots. Plants vary considerably in the amount of Si that they accumulate, with grasses and cereals being particularly high and most dicotyledons being much lower. Once in the plant Si is transported in the xylem and most of it is deposited in the leaf cell lumina and walls as solid amorphous silica. These deposits are commonly known as phytoliths.
Plant scientists do not consider Si to be an essential element, and most plants can complete their life cycles in its absence. However, Si is important in defence against grazing, pests and pathogens. It also has roles in the amelioration of abiotic stresses such as aluminium and heavy metal toxicity. Silicon fertilisation is very important for rice, and is also used for other crops such as sugarcane on occasions, but is rarely employed in Northern Europe. There are, however, some concerns that soluble Si availability may become depleted in some situations where plant amorphous silica is not returned to the soil. Most recently there has also been interest in the potential of phytoliths to sequester carbon in soils thereby helping to combat climate change.
This paper will describe the availability of Si in soils, the uptake and deposition of Si within plants, and the roles of Si in decreasing abiotic and biotic stresses. It will then focus on the management of Si in soils, worries about decreased Si availability in the future, and the possibility that phytoliths may be important for carbon sequestration.