Si fundamentals

Silicon measures

Si concentration: often reported as mg Si per g dry plant weight (mg/g) or as percentage silicon by dry weight (%)

Si content: total amount of silicon in the plant (e.g. mg/plant).

Plant available silicon: the amount of Si in the substrate that is available for uptake by plants, as mg Si per kg soil (mg/kg). Often measured in soils by extraction with CaCl2 (but other methods also, see below) and important to include for manipulative experiments and to allow comparisons among studies/systems.

Glossary

Amorphous silicon (ASi): hydrated form of silica (SiO2.nH2O). When used to describe soil silicon, includes silica from both silicon-containing minerals and from biological material, e.g. phytoliths.

Biogenic silicon (BSi): most commonly used to describe silica in soils that originated from organisms, e.g. phytoliths.

Biomineralisation: the process by which living organisms produce minerals (e.g. by precipitation, dehydration).

Dissolved silicon (DSi): used to describe silicon in the soils in the form of monosilicic acid (and so is available to plants).

Pelagic dissolved silicon (PDSi): monosilicic acid in the ocean.

Phytolith: from the Greek “phyto” for plant and “lith” for stone. These are silica-filled structures in plants that take a variety of different forms. Also known as silica cells.

Silicification: process by which an organism incorporates soluble silicic acid Si(OH)4 in the form of polymerised insoluble hydrated silica (SiO2.nH2O).

Chemistry

Silicon is able to form a wide variety of different chemical compounds. The most relevant when examining silicon in plants are:

  • Silicon dioxide (SiO2): also known as silica. Hydrated silica is the form that plants deposit silicon in e.g. silica cells. It is also a common form in many rocks including granite but plants are not able to absorb silicon from the soil in this form.
  • Monosilicic acid (SiOH4): this is the form of silicon that plants are able to take up from the soil. Also known as orthosilicic acid.
  • Polysilicic acid (SiOH4)n: formed from the polymerisation of monosilicic acid, plants are not able to take up silicon in this form.

Methods

Methods for measuring silicon in plants and the silicon available to plants are not yet standardised. However, some methods are more frequently used and relevant papers are listed here. Some example methods are also listed. Please get in touch here if you think there are papers missing or you have a method that you would like to include.

Applying silicon

Different sources and concentrations of silicon are commonly used for different experiments. However, some silicon applications are more common than others, for example:

  • Hydroponics: growing plants in 2 mM silicon from either dissolved sodium/potassium silicate or silicic acid (produced by passing silicates through a cation-exchange resin) is one of the most common experimental designs. It is important to adjust the pH and balance the cations between silicon-treated and untreated plants.
  • Soil: as with hydroponics, 2 mM dissolved sodium/potassium silicate is quite common, although the total amount of silicon applied in each experiment is very variable and depends on the size of the pots / field the plants are grown in.

Plants

  • Reidinger, S., Ramsey, M.H. and Hartley, S.E., 2012. Rapid and accurate analyses of silicon and phosphorus in plants using a portable X‐ray fluorescence spectrometer. New Phytologist, 195(3), pp.699-706. https://doi.org/10.1111/j.1469-8137.2012.04179.x
  • Puppe, D., Kaczorek, D., Buhtz, C. and Schaller, J. (2023). The potential of sodium carbonate and Tiron extractions for the determination of silicon contents in plant samples—A method comparison using hydrofluoric acid digestion as reference. Frontiers in Environmental Science, 11 (May), pp.1–9. https://doi.org/10.3389/fenvs.2023.1145604

Soils

  • Georgiadis, A., Sauer, D., Breuer, J., Herrmann, L., Rennert, T. and Stahr, K., 2015. Optimising the extraction of amorphous silica by NaOH from soils of temperate-humid climate. Soil Research, 53(4), pp.392-400. https://doi.org/10.1016/j.envpol.2016.04.008
  • Sauer, D., Saccone, L., Conley, D.J., Herrmann, L. and Sommer, M., 2006. Review of methodologies for extracting plant-available and amorphous Si from soils and aquatic sediments. Biogeochemistry, 80(1), pp.89-108. https://doi.org/10.1007/s10533-005-5879-3
  • Wu, W., Limmer, M.A. and Seyfferth, A.L., 2020. Quantitative assessment of plant‐available silicon extraction methods in rice paddy soils under different management. Soil Science Society of America Journal, 84(2), pp.618-626. https://doi.org/10.1002/saj2.20013