Zeolites are natural alumino-silicates formed from volcanic eruptions. They are hydrophilic, meaning they will absorb water. The process is reversible, so they gain from and lose water to the environment.

Zeolites have a much different structure than clay minerals. Aluminium, silicon and oxygen are arranged in a regular, micro-porous framework of tetrahedral units. These pores and interconnected voids contain cations and water molecules, and together with the huge surface area make zeolites extremely effective ion and cation exchangers. Cations can be exchanged by ion exchange and water can be removed reversibly by application of heat.

The porous structure can trap and release nutrients like ammonium, potassium, and calcium. They are particularly effective in retaining ammonium ions and releasing them slowly to plants.