PineIslander,
another natural source of available Silicates is Australian Pine mulch. The phytoliths of Silicates in the wood, along with a lot of resin, are what destroy lumber-mill saws, so operators don't want it.
Your sandy ground is mostly Silicates, un-available until in contact with acids, such as from decaying vegetation, or such as from acid-forming fertilizers, sulfates.
Despite lack of essentiality of Silicon for growing inside laboratories--- where there is no wind, growing Grass-Family crops in Silicon-deficient conditions is un-profitable. Silicate fertilizers, or acid applications to soil can make a huge difference for Sugar-Cane and Rice, and other grains, and also in Cucurbits and Potatoes. It can be a major nutrient--- a structural nutrient.
With broad-leaf trees, Silicon is apparently a micro-nutrient, or even a trace nutrient. In most cases it has not been studied at all.
Potassium Silicate is not compatible with many (most?) other spray ingredients--- most certainly not compatible with any ingredients requiring a notably acidic mix. In hindsight, it appears that many of my spray mixes were useless from the moment of mixing--- it is a clear substance, so my jar tests didn't always show a visible problem.
My attempts failed to show any benefit from using Dyna-Gro Pro-TeKt liquid fertilizer on mango, probably mostly due to the above-mentioned tank-mix incompatibilities, and perhaps because my jug has been sitting around in my garage for 20 years!
I will be re-visiting the matter.
I attended a continuing education class for commercial grove spraying, last summer, where Dr. Jonathon Crane handed out an Aug 2017 "Updated list of fungicides labeled for Florida mango production." It listed Potassium silicate (not classified in any fungicide group), brand name Sil-Matrix, allowed for organic production, "disease depressant."
Field trials in the last four years, in New Zealand and in California, have shown impressive results in eradicative / curative spray mixes on grapes already infested with powdery mildew. [The powdery mildews of grapes and mangos both tend to be described as "Oidium spp." The powdery mildews of most other crops are not at all closely related.]