ReviewThe nature of biogenic Si and its potential role in Si supply in agricultural soils

- Biogenic Si is the major pool of Si through which Si is cycled in soils under natural ecosystems.
- For arable crops, residue returns recycle large quantities of phytolith Si back to the biogenic pool of soil Si.
- Under grazed pastures the bulk of phytolith Si is recycled back to the biogenic soil Si pool.
- Use of a soil test to extract the biogenic Si pool (alkali extractant) would improve estimation of potentially available Si in many agricultural soils.

Although the biogenic pool of Si in soils is known to be of central importance to plant uptake and Si cycling in natural forest and grassland ecosystems, its role in agricultural systems is controversial and unclear. The biogenic pool is mainly composed of phytogenic (plant-derived) amorphous silica (deposited in plant shoots as phytoliths) but there are also minor components of zoogenic, microbial and protistic silica. In natural ecosystems the pool of biogenic Si in the soil is typically several orders of magnitude greater than annual plant uptake so slow dissolution of this Si pool supplies the plant with Si (as silicic acid) while litter fall replenishes the pool with newly-formed phytolith Si. However except for grazed pastures, such cycling of Si is much decreased under agriculture because phytolith Si is removed from the field in harvested products and crop residues. For graminaceous crops, which commonly accumulate Si and are Si responsive (e.g. rice and sugarcane) only about 20% of accumulated Si is present in harvested products (e.g. harvested grain or cane) and the remaining 80% is present in harvest residues (straw or cane trash). The extent of phytolith Si removal, and thus rate of diminution of the biogenic pool of soil Si, is therefore greatly dependant on the magnitude of Si uptake by the crop and whether crop residues are retained or removed. Where crop residues are regularly returned to the soil, and for pastoral soils, biogenic Si will remain a significant source of potentially available Si. Thus, in addition to routine soil tests for Si using neutral salt or acidic reagents, an additional broad estimate of biogenic Si (e.g. alkali-soluble Si) is likely to improve evaluation of potentially plant available Si in many agricultural soils.