As a preemptive sidebar, lots of modern melon and cucumber varieties have relatively small root systems and stem diameters, and are not great at feeding biology large quantities of sugars, thus developing a much smaller symbiotic microbiome, which is so important for releasing silicon. The reason seedless watermelon grafted on to gourd rootstock produces such a yield response has less to do with disease resistance and more to do with root system and stem size in my opinion.

Possible sources of silicon are Wollastonite, (calcium silicate) and rice hulls. We also see some interesting responses from including diatomaceous earth in the media. Include a strong biological component in the media for silicon release.