A contemporary overview of silicon availability in agricultural soils

Our current understanding of silicon (Si) availability in agricultural soils is reviewed and knowledge gaps are highlighted. Silicon is a beneficial rather than essential plant nutrient and yield responses to its application have been frequently demonstrated in Si-accumulator crops such as rice and sugarcane. These crops are typically grown on highly weathered (desilicated) soils where soil solution Si concentrations are low. Increased yields are the result of simultaneous increases in plant tolerance to a wide range of biotic (plant pathogens, insect pests) and abiotic (water shortage, excess salts, metal toxicities) stresses. Traditionally, soil solution Si is viewed as being supplied by dissolution of primary and secondary minerals and buffered by adsorption/desorption of silicate onto Al and Fe hydrous oxide surfaces. In recent years it has become recognized that phytogenic cycling of Si [uptake of Si by plants, formation of phytogenic silica (SiO2 center dot nH(2)O) mainly in leaves and subsequent return of this silica to soils in plant litter] is the main determinant of soil solution Si concentrations in natural forests and grasslands. Considerable diminution of the phytogenic Si pool in agricultural soils is likely due to regular removal of Si in harvested products. A range of extractants (unbuffered salts, acetate-based solutions, and acids) can provide valuable information on the Si status of soils and the likelihood of a yield response in rice and sugarcane. The most common Si fertilizers used are industrial byproducts (e.g., blast furnace slag, steel slag, ferromanganous slag, Ca slag). Since agriculture promotes soil desilication and Si is presently being promoted as a broad spectrum plant prophylactic, the future use of Si in agriculture is likely to increase. Aspects that require future research include the role of specific adsorption of silicate onto hydrous oxides, the significance of phytogenic Si in agricultural soils, the extent of loss of phytogenic Si due to crop harvest, the role of hydroxyaluminosilicate formation in fertilized soils, and the effect of soil pH on Si availability.