Calcium silicate (wollastonite) in soilless crops

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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.

Vansil CS-1, one of the most common forms of calcium silicate (wollastonite) used as an amendment in soilless crops.

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

  1. 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).
  2. Dose conservatively, then verify with tissue Si or leachate pH before scaling. Tomatoes show rate-sensitive physiology (3).
  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

CropMedium and Si sourceApplication ratePositive effects on yield or qualityReported negativesRef
TomatoSubstrate mix, calcium silicate among Si sourcesField-equivalent 0 to 800 kg SiO₂ ha⁻¹ mixed pre-plantImproved postharvest durability and maintained physicochemical quality vs control; effect depended on dose and sourceNone specified at optimal rates(2)
TomatoSubstrate, calcium silicate mixed before transplant0, 150, 300, 450, 600 kg ha⁻¹Reduced gas exchange and chlorophyll at midcycle at higher rates, indicating potential performance penalty(3)
TomatoSubstrate, silicon sources including calcium silicateMultiple ratesYield responded to Si fertilization depending on source and rate(4)
CucumberSoilless substrate, wollastonite3 g L⁻¹ of substrate under 75-85% container capacity+24.9% yield vs untreated; fruit size and soluble solids unchangedNone noted at that rate(5)
CucumberSubstrate drench, calcium silicate solution50-100 mg L⁻¹ SiO₂ applied to substrateBiomass gains under specific moisture regimes; quality unchangedNo quality gain at tested doses; response moisture-dependent(6)
AnyPeat or coir mixes, wollastonite~1 g L⁻¹ media typical in studySteady Si release over months supports long cropsRaises 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.

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