Use for soil mixes, cuttings, seed starting and more! Vermiculite has multiple purposes around the greenhouse. Add to soil mixes for additional water retention. Vermiculite also contains potassium, magnesium, and calcium to help avoid mineral deficiencies. Cover your seedling trays with vermiculite to help maintain moisture levels. Vermiculite has shown the capacity to absorb some fertilizers, helping to reduce the frequency of fertilizing.