Vermiculite and perlite have different chemical origins but similar characteristics. For the gardener, they serve identical purposes. Both are wonderful soil conditioners. Vermiculite loosens soil and maintains moisture while, at the same time, aiding drainage. Perlite does not retain moisture as well as vermiculite, but aids drainage equally well. While some consider these products expensive to use in raised beds or gardens, greenhouse and indoor gardeners rely on vermiculite or perlite.
Find vermiculite at garden supply stores and some hardware stores. Vermiculite used to be easy to find in large quantities but that is no longer the case. Garden supply stores still sometimes offer bags of, for example, 4 cubic feet or less of vermiculite.
Mix up to 10 percent of vermiculite into potting soil. Use higher percentages of vermiculite with more clay-like soils (such as a full 10 percent.) Take into account what you will be planting in the soil. For example, tomatoes need soil that drains well. Add a full 10 percent of vermiculite even to loose potting soil if planning on growing tomatoes in pots.
Sprinkle a dense layer of vermiculite on the garden in the fall. Do this on a dry day. Rake gently with a dirt rake to mix the vermiculite in with soil. The more clay-like the garden soil, the more helpful the vermiculite will be.
Include dry leaves and compost, if you have it, when mixing the vermiculite in with the soil.
Notice that your crops do better in the spring if you tend to the soil this way in the fall.
Mix 10 percent of vermiculite in with soil before filling raised beds. Mix in whatever food or fertilizers you want in the raised bed at the same time you mix in the vermiculite.
Some gardeners choose to sprinkle a dense layer of vermiculite on the bottom of the raised bed before adding soil. This optional step does not replace mixing vermiculite into the soil before filling the raised bed. The purpose is to create a boundary with great drainage between the bottom of the raised bed and the ground underneath the weed block. Weed block is the material on the bottom of the raised bed.