Silicon in media is not a magic switch. In soilless systems it can help, it can do nothing, and at the wrong rate or pH it can hurt. Calcium silicate sources such as wollastonite release plant-available Si into inert substrates and typically raise pH, which is useful in peat but potentially more risky in coir or already alkaline systems. A recent substrate study quantified this clearly: wollastonite steadily released Si for months and increased media pH about 0.5 to 1 unit depending on substrate composition (1). With that in mind, here is the evidence for tomatoes and cucumbers grown without soil, focusing only on media or root-zone applications.
Tomatoes
Two independent Brazilian groups that amended substrate with calcium silicate found quality benefits but also rate-sensitivity. In a factorial test across Si sources and doses, calcium silicate treatments improved postharvest durability and maintained physicochemical quality of fruits; the effect size depended on the source and the dose used (2). A protected-environment pot study that mixed calcium silicate into the substrate before transplanting reported reductions in gas exchange and chlorophyll at midcycle at higher rates, a warning that more is not always better (3). Earlier yield work that compared sources also detected response to silicon fertilization in tomatoes, but the magnitude varied with rate and material (4).
Cucumbers
When wollastonite was incorporated into the soilless substrate, 3 g L⁻¹ increased yield by ~25% under moderate moisture restriction, with no penalty to soluble solids or fruit size. Lower doses or excessive irrigation did less (5). A separate work that applied a calcium-silicate solution into the substrate showed small gains in biomass under specific moisture regimes and no change in soluble solids, again pointing to context and dose as the deciding factors (6).
Practical takeaways for media use
- Treat calcium silicate like a weak liming Si source. Expect a pH rise. In peat this can be helpful, in coir or high-alkalinity waters it can push you out of range (1).
- Dose conservatively, then verify with tissue Si or leachate pH before scaling. Tomatoes show rate-sensitive physiology (3).
- Target crops and situations with the strongest evidence. Cucumbers under moderate moisture restriction and strawberries in organic substrates show the clearest yield and quality benefits (5), (7).
Summary table – media or root-zone Si only
| Crop | Medium and Si source | Application rate | Positive effects on yield or quality | Reported negatives | Ref |
|---|---|---|---|---|---|
| Tomato | Substrate mix, calcium silicate among Si sources | Field-equivalent 0 to 800 kg SiO₂ ha⁻¹ mixed pre-plant | Improved postharvest durability and maintained physicochemical quality vs control; effect depended on dose and source | None specified at optimal rates | (2) |
| Tomato | Substrate, calcium silicate mixed before transplant | 0, 150, 300, 450, 600 kg ha⁻¹ | – | Reduced gas exchange and chlorophyll at midcycle at higher rates, indicating potential performance penalty | (3) |
| Tomato | Substrate, silicon sources including calcium silicate | Multiple rates | Yield responded to Si fertilization depending on source and rate | – | (4) |
| Cucumber | Soilless substrate, wollastonite | 3 g L⁻¹ of substrate under 75-85% container capacity | +24.9% yield vs untreated; fruit size and soluble solids unchanged | None noted at that rate | (5) |
| Cucumber | Substrate drench, calcium silicate solution | 50-100 mg L⁻¹ SiO₂ applied to substrate | Biomass gains under specific moisture regimes; quality unchanged | No quality gain at tested doses; response moisture-dependent | (6) |
| Any | Peat or coir mixes, wollastonite | ~1 g L⁻¹ media typical in study | Steady Si release over months supports long crops | Raises media pH by about 0.5-1 unit depending on substrate | (1) |
Bottom line
Use calcium silicate where the crop and context justify it, not by default. For cucumbers and strawberries the upside on yield and quality is most consistent when Si is in the root zone. For tomatoes, treat calcium silicate as a quality tool with a narrow window and verify plant response; higher rates can backfire physiologically. If you want to try calcium silicate, mix wollastonite with your media at a rate of 3g L⁻¹, then test the effect on pH and Si in tissue.
