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A firm favourite with keen gardeners is vermiculite and when you want to retain water then choose to add vermiculite to your chosen media.
Vermiculite is a natural, sterile soft mica mineral with great water holding and insulation properties. It also offers a high cation exchange capacity which means it will slowly release nutrients back into your plant which it has absorbed during watering. Using Plant It Vermiculite can also aid propagation when used as a top layer as it reduces evaporation and can help ensure the seed case is lost.
Using Vermiculite also reduces plant stress in extreme weather conditions due to its insulation properties.