Silica is most readily absorbed through foliar application.
But I regularly use it as an additive to my nutrient solution as well. I think it’s best to alternate between the two.
I usually hold off on soil drench until I’m at full strength nutes so the pH won’t have to be adjusted.
Silica will raise your pH substantially. So check to see how much you can add without pushing it above 6.5.
My favorite at the moment is Simply Silica by Supreme Growers.
Use it at 8ml/gal for both soil drench and foliar application. Foliar once per week. Soil application with every watering.
Do not use silica products past the second week of flowering.