Silicic Acid in Cannabis – Uptake, Deposition in the Tissue, and the Question of the Source

Silicic acid is widely regarded in cultivation as a means of producing sturdier stems and reducing fungal pressure. The long-standing objection that cannabis does not take up silicon anyway is outdated. What does persist and continues to mislead is the equation of silicon content with plant availability.

Uptake and deposition

Cannabis is not a non-accumulator. Aquaporins of the NIP2 type have been identified as candidates for transport, and deposition has been localised in bast fibres and non-glandular trichomes. The species is therefore classified as an intermediate accumulator with active, transporter-mediated uptake.

Silicon is taken up through the root as dissolved silicic acid and distributed with the transpiration stream. Inside the plant it is immobile and is deposited as amorphous silica beneath the leaf cuticle and in the cells of the leaf epidermis. Where it is deposited, it stays.

Why the source is what matters

The silicon content of a product says little about its effect. Only dissolved silicic acid is available to the plant, and the solubility differs between sources by orders of magnitude. In a comparison of several silicon sources, alkaline sources such as calcium silicate slag delivered substantially more plant-available silicon and higher uptake than sources without a pH-correcting effect; the latter included potassium silicate and diatomaceous earth.

That a silicon application reaches the plant at all is documented in existing work: a silicate treatment raised the leaf silicon content of hemp by a factor of 2.1.

Rock dusts are a special case. Silicate agrominerals release calcium, magnesium, potassium, and silicon through gradual weathering – over months to years, not weeks. As a silicon source for a running crop they are therefore of little use, but as a long-term soil amendment they are.

Horsetail

Field horsetail is the strongest silica former among the native plants; silicate is found there in cell walls, cell plates, plasmodesmata, and stomata. The total content is around 5 % silicon in the whole plant.

What matters, however, is the preparation: the maximum water-extractable silicon amounted to only 0.3 % of the plant material in an unboiled infusion. The lion’s share is present as polymerised biogenic silica that a cold preparation does not dissolve. The traditional recommendation to boil the herb for five to fifteen minutes thus has a measured basis.

It should also be noted that a horsetail preparation contains not only silicic acid but also flavonoids, phenolic acids, and saponins. An observed effect against fungal infection therefore does not necessarily originate from the silicon.

Foliar spray or root application

The difference is larger than the labels suggest. With a root application the root takes up the silicic acid and the transpiration stream distributes it throughout the plant. With a foliar spray the pathway through the cuticle is poorly understood, and the applied silicon barely moves from the point of contact. A foliar treatment works where it lands, not inside the stem.

For the disease side this is still useful: trials link root-applied silicates to increased leaf silicon content and reduced powdery mildew severity.

Relevance for propagation

Whether a silicic acid supply improves the rooting of cuttings has not been measured in cannabis. A precedent exists in another species: cuttings on the propagation bench, watered regularly with potassium silicate, showed more roots, longer primary roots, and greater root mass in nearly all tested cultivars compared to untreated controls. This is not directly transferable, however, because silicon also influences lignin formation, and a more heavily lignified cutting base roots less readily in some species.