Succulents are plants with thick, waxy leaves that store extra water to keep the plant alive during drought. They grow in many areas of the world that are dry at least part of the year. The roots are as specialized as the leaves, fleshy and adapted to airy, quickly draining soil--conditions that need to be imitated to grow them in containers.
The primary consideration in buying or mixing a potting soil for succulents is drainage. Water needs to be able to move through the soil quickly. The soil should dry out quickly, returning to a state where spaces in the soil are filled with air, not water. The elements that make most potting soils desirable--richness in nutrients and moisture-holding capacity--are not useful for succulents. If the soil stays moist for longer than a few days, especially in winter, the roots will rot.
Commercial potting soil, the ordinary kind you'd use for any plant, can be used but it needs to be mixed with non-organic materials such as coarse builder's sand, perlite, crushed granite or small gravel. Instead of potting mix, use soil out of the garden if you sterilize it by heating to 180 degrees for half an hour (see Resources for directions.) Very fibrous organic materials like peat moss and fine bark may be used since they increase the airiness of the soil. Be sure to let the plants dry out between waterings, however.
Mix any of the following ingredients together and then test to make sure the soil is loose enough by squeezing it in your hand. If the clod crumbles quickly without holding a shape, it's the right mix. Use one part garden soil or regular potting mix, one part pumice and one part peat moss; or use one part potting mix and one part pumice; or use two parts potting mix, one part pumice and one part builder's sand or other non-organic material such as builder's sand.
Succulents often grow in soils that are poor in nutrients and need little fertilizing. It is useful, however, to add bone meal, which is high in phosphorous. Mix 1/2 cup into each 3 gallons of your potting mix.
Use clay pots whenever possible. These dry out more quickly than plastic or glazed ceramic. Repot succulents infrequently since they grow well in shallow, limited spaces. When you do repot, let the plant sit in the dry soil mix for two or three days before watering. This gives the roots time to heal. If watered right away, the treatment most plants need, the roots may rot.