Vitax 20L Perlite Micro-Porous Granules Soil ImproverPerlite is a naturally occurring volcanic rock which, when heated to high temperatures, produces lightweight micro porous granules. When mixed with compost, Perlite holds water and nutrients in the compost, producing a healthier growing media. Perlite: Mix with soil when planting out to help hold water and nutrients close to the root system Use alone for seed germination Improves water and nutrients holding capacity when mixed with compost. Primarily