Vermiculite is a hydrated silicate mineral that, when heated, expands into light, accordion-like flakes. Unlike perlite, it soaks up water and has a high cation exchange capacity, so it holds potassium, calcium, magnesium and ammonium on its surfaces. It is used in seed-starting mixes and in soils where more water holding is wanted.
In practice
Growers add 10-20% vermiculite to sandy or very open mixes, or use it in seed starting and cloning mixes. Too much in a container mix reduces air space and can make the medium stay wet for long periods. Some older vermiculite from specific mines was contaminated with asbestos; modern horticultural grades are tested, but avoid breathing dust from any dry mineral product.
Don't confuse with
- Perlite: adds air and drainage; vermiculite adds water and nutrient holding.
- Mica: the related mineral family; not used directly as a medium.
Perlite is a light, white, puffed volcanic glass added to growing mixes to increase drainage and air space; it is chemically inert and holds no nutrients.Cation exchange capacity (CEC) is a medium's ability to hold positively charged nutrients such as calcium, magnesium and potassium on its surfaces and release them later.Ammonium (NH4+) is the positively charged form of nitrogen; a small share stabilises root-zone pH, but too much reduces cannabis yield and quality.Cloning is propagating a plant by rooting cuttings from it, producing new plants that are genetically identical to the donor.Potting mix is a manufactured container medium, usually peat or coco with perlite, lime and starter fertiliser, designed to drain and hold air better than garden soil.Air-filled porosity is the share of a container's volume that holds air rather than water after it has drained, a key factor in root oxygen and overwatering risk.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.Field capacity is the amount of water a medium holds after it has been saturated and allowed to drain freely; in pots it is called container capacity.
