The name ‘Silicon’ is often used as a symbol (pars pro toto) for many different silicon compounds used in agriculture, aquaculture and as dietary supplement for humans and animals. Silicon as such, is a chemical element with the symbol Si and atomic number 14.

This recognition of the importance of silicon is now based on the results of extensive research showing the many beneficial effects, mentioned in recent reviews (1;2):

1. Improved growth and more biomass, due to an increase of the root mass.
2. Increased nutrient uptake, like calcium, phosphorus, potassium, and zinc.
3. Increased chlorophyll content and photosynthesis.
4. Increased strength and rigidity making plants more resistant to physical stresses such as wind/storms reducing the risk of lodging (stem bending or breaking).
5. Improved drought and heat tolerance.
6. Reduction of other abiotic stresses.
7. Enhanced resistance to pests and diseases, reducing the need for protective chemicals.
8. Increase in quality with longer shelf life.
9. Improved soil structure.

The importance of Silicon for ‘Life on Earth’

Silicon plays an important role in Life on Earth: bio-silicification, the formation of biological structures composed of silica, and is widely distributed among eukaryotes.

For example: diatoms. These micro-algae in waterways and oceans, form a significant part of the Earth’s biomass. Diatoms are an important sink for CO₂ and absorb around 35% of all atmospheric carbon dioxide, while at the same time producing over 20% of all oxygen on the Earth, and almost half of the organic material in the oceans.

Diatoms need silicon for their growth: the cell walls of diatoms (called frustules) are composed of silica being absorbed from the water as monosilicic acid (MSA): Si(OH)₄ or H₄SiO₄), the only bioavailable silicon compound. Without silicon, there would be no diatoms at all, with disastrous consequences for Life on Earth.

The uptake of MSA into the cell interior is an active process facilitated by membrane proteins,  SIT’s. This active transport is needed because the very low concentration of MSA in all water systems (3; 4). When extra MSA is added, diatoms bloom, resulting in higher yields of shrimps (5) and fish (6). It has also been shown that MSA-limitation in oceanic water systems, facilitates virus infections and mortality in diatoms including brown algae (Phaeophyta) (7).

Almost all other aquatic organisms, such as seaweed, pondweeds (Potamogeton) and water milfoils (Myriophyllum) contain various levels of silicon. The ability of seaweeds and aquatic plants to accumulate silicon has been of interest in various fields, including agriculture and biotechnology, because its implications for plant growth, stress tolerance, and ecological interactions in aquatic ecosystems. The uptake of Si by aquatic vegetation is not only an important transient sink for Si in the global biogeochemical Si cycle, but is also important for  carbon turnover in aquatic ecosystems as well (8).

Silicon compounds and silicic acid

Silicon (Si) is the second most abundant element on earth (28%) mainly occurring as silicon dioxide (silica: SiO₂) in the form of quartz, and in combinations with other minerals. Compared to carbon (0.02%), silicon is almost 1400 times more abundant. Silicon in soils is grouped into solid, adsorbed, and liquid phase fractions (9).

Silicon is primarily present in the solid phase fraction, that occurs naturally in minerals such as quartz, feldspars, micas, and clays. The adsorbed and liquid phase fractions of Si consist of silicates and (poly)silicic acids, from which monosilicic acid/orthosilicic acid (MSA) can be released, the only plant available silicon compound.

Another source of MSA consists of organo-silicon compounds, biogenic silica in phytoliths, in the remains of plants and trees. Due to decomposition process, MSA can be gradually released in the soil through weathering and dissolution.

The concentration of MSA is low, compared to other dissolved ions. Monosilicic acid is very unstable, because of its high tendency to polymerize to oligomers, polysilicic acid and ultimately to amorphous silica (SiO₂). Due to this instability, the concentration in the soil is low, often too low for optimal plant growth resulting in a silicic acid deficiency in plants (10).

In native soils the concentration of MSA is higher compared to agricultural soil soils, ranging from < 0.1 to 0.6 mM / 2 to 18 ppm (11).

Polysilicic acids influence the physical properties of soils; they can link to soil particles, which improves soil aggregation and water-holding capacity, being beneficial in case of drought.

Silicon fertilization can increase soil exchange capacity, improving the soil’s water capacity, and other beneficial effects. All these effects are due to the change in soil mineral composition that results from silicate addition (Si fertilizers) and/or the formation of new clay minerals, with high biogeochemical activity. They have large surface areas and can adsorb water, phosphates, potassium (K), nitrogen (N), aluminium (Al), and heavy metals (12).

The uptake of monosilicic acid, distribution, and the silicon content in plants

Monosilicic acid is absorbed by plant roots through passive diffusion or active transport. In the case of passive uptake, the silicon content in the plant is lower compared to plants with an active uptake. In most monocots, such as rice, MSA is absorbed by an active transport facilitated by membrane transporters, like the uptake of MSA in diatoms. In other plant species (mostly dicots), the uptake is the result of diffusion leading to a lower Si content of dicots. Based on the Si content, plants are classified as a) high accumulating plants (like rice, wheat, barley, bamboo, horsetail, and sugarcane) with 1-10 % dry weight Si (= 10 to 100 g kg¹); b) intermediate accumulators with 0.5-1% Si; c) low accumulators < 0.5 % Si (13).

The silicon content and availability vary, depending on several factors, including silicon availability in the soil, plant species and cultivars, growing conditions and agricultural practices including the use of NPK fertilizers and phytosanitary products that have a negative impact on the MSA concentration in the soil solution.

Once inside the xylem, MSA moves upwards in the xylem and is distributed to the root, stem, tillers, leaves, and reproductive structures. Here silicic acid precipitates as (amorphous) silica. These depositions, called phytoliths, replicate the structure of the cells providing structural support and strength, making the plant more resistant to physical stresses like wind/storm preventing lodging, especially in monocots. Due to the increased strength, plants are also less susceptible to diseases, such as fungal infections, and pests. Additionally, silicon enhances tolerance to other abiotic stresses such as drought, heat, and metal toxicity.

Overall, the presence of silicon in plant tissues contributes to enhanced plant resilience and stress tolerance.

Silicon fertilizers and types of application

Agriculture and long-term applications of traditional NPK fertilizers can deplete the silicon content in the soil. If crops, especially the high Si accumulating plants, such as rice, sugarcane, grasses, are continuously grown without silicon supplementation, the concentration of MSA in the soil will be further depleted.

In case of excessive nitrogen (N) application, particularly in the form of ammonium-based fertilizers, the soil acidity will increase. Acidic conditions can decrease the release of MSA from other silicon compounds, further reducing its availability for plant uptake.

So, there is a rationale for the use of silicon fertilizers, not only for silicon-accumulating crops (14), but other crops will also benefit.

These Si fertilizers should fulfil several criteria: a relevant Si content, suitable physical properties, availability, cost-effectiveness, and absence of polluting substances/heavy metals (2).

Solid silicon compounds

Si sources, such as calcium silicate (slag), potassium silicate, sodium silicate, quartz sand, rice hull ash, diatomaceous earth, amorphous silica and biochar, have high a silicon content, but are hardly plant available. Nevertheless, when larger amounts are applied (0,5-4 tons/ha), positive effects are seen, also due to soil improving properties. Because of the large quantities applied, relevant amounts of MSA become available despite the (very) low conversion rate into MSA.

Liquid silicon compounds

Liquid silicon fertilizers can be divided into: (1) silicates; (2) stabilized silicic acid; and (3) silica nanoparticles (15).

In the last 3 decades liquid silicates have been used, such as Na-silicates, K-silicates, Ca-silicates and Diatomaceous Earth (DE). The application of silicate sprays reduced the infection rate, while DE sprays, often in combination with DE soil application, have positive effects on growth and yield without significant effects on biotic stresses such as pest and disease incidence (16). Liquid (stabilized) silicic acid (MSA and oligomers) has been used since 2003. Due to foliar application, smaller quantities are needed (2-3 L/ha/crop cycle), also because it is almost pure MSA. Stabilized MSA can also be applied hydroponically and by drenching (17). Liquid MSA induces a larger root system resulting in enhanced nutrient uptake, increased growth parameters and yield with higher quality. Moreover, MSA-sprays also decrease abiotic as biotic stresses.

In the last decade, nano-silica particles have also been introduced. Nanoparticles are small-sized particles ranging from 1 to 100 nm that possess enhanced physicochemical properties compared to the bulk material (18). Sprays with silica nanoparticles also induce positive effects on growth and yield, abiotic stresses and can decrease infection rate. The ultimate effectiveness requires further studies.