Amorphous silica is a form of silicon dioxide (SiO₂) that lacks the highly ordered crystal structure found in crystalline silica such as quartz. Instead, its silicon and oxygen atoms are arranged in a disordered, glass-like network. Common examples in horticulture include diatomaceous earth, rice hull ash, silica gels, volcanic glass, and many industrial silicon products.
In soils and growing media, amorphous silica is generally more reactive and more soluble than crystalline silica, although it still dissolves slowly. As it weathers, a small fraction can release monosilicic acid (H₄SiO₄), the form of silicon that plants absorb through their roots. The rate of dissolution depends on particle size, pH, temperature, moisture, and the specific type of amorphous silica.
Not all amorphous silica sources are equally available to plants. Freshly precipitated silica and some silicate fertilizers release monosilicic acid relatively readily, while diatomaceous earth and silica sand release it much more slowly. Even though diatomaceous earth is classified as amorphous silica, only a tiny portion dissolves at any given time, with the vast majority remaining as solid particles.
Amorphous silica is distinct from crystalline silica. Crystalline forms are more chemically stable and release plant-available silicon much more slowly under normal growing conditions.
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