Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Pure soil, with mineral particles called sand, silt and clay are good. Loam is a combination of sand silt and clay in proportions that make the soil easy to work with in the garden. My info is based on US soil service designations, I do not know if they are the same in the UK.
Organic matter, compost, sawdust, peat moss or any similar product is not generally good when you are going with a topsoil sort of substrate. A small amount is not a problem, but more than 25% is way too much. Makes better garden soil, but not aquarium soil.