From Only Ornamental when I ask him
]Last time I checked there was just one family of pores (recently discovered aquaporin-like proteins) which allow silicic acid to enter plant roots with subsequent xylem loading. On one hand, these pores have nothing to do with other nutrients and on another, ortho
-silicic acid though being an acid is so weak that it only exists in its neutral form at a physiological pH. Most likely, silicic acid enters plants by mass flow, passive diffusion, and the transpiration stream (can't remember the proper term for the latter right now, sorry).
Couldn't find anything else regarding further interactions of silica with binding proteins, transporters, or pores. If memory serves me right, silicic acid is deposited mostly outside the cells and solely as silica (i.e. polymerised silicic acid). BTW silicic acid does not form salts within plants or in soil, potassium silicate etc. are high temperature products either of industrial processes or volcanic activities which, once dissolved in water, turns at best into neutral silicic acid and potassium hydroxide.
Silicon is a beneficial element for most plants and essential for a few such as rice or horsetail and helps in plant defence and stress resistance. One mode of action is simply the mere presence of silica deposits; would you eat an apple spiked with needles or gravel? Bout the same for lice and grasshoppers chewing on grass leaves
Regarding the negative charge: Yes, silica particles have a slight negative outer charge but that charge is nowhere near what anions and cations have but rather a soft shimmer on the surface of aggregates of hundreds and thousands of atoms (called zeta potential) and not that hard and constant charge on every single atom. The slight negative charge contributes for example to soil CEC and inhibits colloid aggregation in silica suspensions, it's also a major contributor for the separation of chemical compounds occurring on a standard TLC plate. Certainly not enough to limit any nutrient uptake. The reason why and how silica sufficiency limits manganese toxicity is still a matter of debate, though.
Is addition of silicic acid beneficial in soilless or hydroculture? Difficult to say cause many products contain not much plant available silica, are rich in additives, or cost a fortune. In addition to that, higher amounts of silica in water precipitate other elements (mostly metals) as insoluble mixed silicates. The net benefit likely depends on growing style, grow medium (many do contain Si by nature), plant variety, material costs, sales value....
I only grow for fun and so far only outdoors in soil and coco/soil/peat mixtures wherein I prefer to add some silica either in the form of Si-rich minerals (silt/loam), montmorillonite/bentonite (I love that stuff!), and/or diatomaceous earth. Most plants seem to love it![/quote]
Zeta potential turns out to be a bitch to chelate