Vermiculite in potting compost gives a very light open compost, holding more water and facilitating re-wetting, thereby lengthening the time between watering. Vermiculite also has excellent ion exchange properties which absorb excess nutrients and release them slowly to the plants via the finest root hairs.
A 50/50 mix of vermiculite and sphagnum moss peat is widely used for greenhouse pot plants and hanging baskets, while a 25/75 mix is generally suitable for for bedding plants, nursery stock, etc.
To improve an existing compost add 20-25% by volume of Vermiculite and mix thoroughly.
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