Vermiculite for gardening purposes. Horticultural-grade vermiculite has the excellent property of improving soil aeration while retaining the moisture and nutrients necessary to feed roots, cuttings, and seeds for faster growth. Vermiculite is clean, odorless, nontoxic, and sterile. It will not deteriorate or turn moldy or rot. Plus, the pH of vermiculite is essentially neutral so it doesn’t change the acidity or alkalinity of your soil.