[QUOTE=Itoero;420558]That makes sence..
Do you know of another silicic acid input?[/QUOTE]

An amorphous silicon dioxide like Diatomaceous Earth would provide a slow release of silicic acid. Faster that a crystalline form, but still pretty slow.

Quick available silicic acid sources would be Pyrophyllitic clay, a natural rare earth source containing largely silicate salts and a wide range of other minerals.
Otherwise Potassium Silicate which is widely available for agriculture and hydroponics.

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