An easy test for drainage and porosity is to fill a six-inch pot with mix, tapping lightly on the container while filling it. Pour one quart of water through the pot. The water should filter through in less than a minute. Additions to Soilless Mixes Fertilizer is the most obvious addition to potting mix. Vermiculite and perlite may also be added to potting mix to add volume without weight.
Calcined clay is a rigid, odorless mineral that resembles cat litter.
Succulents and alpines that require excellent drainage can be grown more successfully if one part calcined clay is added to two parts native soil. If incorporated in large enough quantities, calcined clay particles can keep a mix loose and aerated allowing a deep, sturdy, and healthy plant root structure to form. In theory, this absorbed water is then slowly made available to plant roots to prevent or delay water stress. Individual particles will absorb between 60 and 400 times their dry weight in water, depending on the specific type. Caution is advised if you are incorporating hydrogels into the potting mix.