Home gardeners use potting soils because they are disease-free, contain no insects or weed seeds and retain air and moisture while draining easily. Various types of potting soil mix are available, so it is wise to check labels before you buy, as some mixes are designed for specific plant types.
Common Potting Soil Mix
Potting soil is generally a soil-free medium and is usually sterilized to kill any potentially harmful organisms. Typically, potting soils are a mix of peat moss, spaghnum moss or coconut coir, with bark and perlite. Perlite is the small white pellets in the soil that look like balls of styrofoam. It is made from volcanic glass that is heated to extremely high temperatures, causing it to pop like popcorn and expand to 13 times its previous volume.
Other Common Ingredients
Potting mixes are not regulated by federal or state standards for their content, so they vary a great deal. Cheaper potting mixes contain styrofoam in place of perlite, which may float to the surface after watering and is likely to compact easily. Some other ingredients that may be included in potting soils are vermiculite, water retention crystals, coarse sand and ground limestone. Fertilizers and additional compost materials are also included in some potting soil mixes. Some of the cheaper varieties of potting mix may even contain waste ingredients or sewage sludge.
Special Potting Soils
Special potting soils are available for specific types of plants. Seed-starting mixes are ideal for cuttings as well as seeds. They contain spaghnum peat moss and higher levels of vermiculite, which increases the retention of moisture and nutrients. African violet (Saintpaulia ionantha) potting mix is a more acidic combination, containing higher levels of peat moss combined with lime, together with perlite, giving better drainage and soil aeration. Cactus potting mixes contain organic materials, sand and perlite mixed with bone meal, to promote growth and blooming.
Mix Your Own Potting Soils
You can easily mix your own potting soils. One simple mix consists of one-third spaghnum peat moss or coconut coir fiber, one-third finely screened compost and one-third vermiculite. You can also customize your homemade potting soils to suit particular plants. Foliage plants thrive in a mixture consisting of two parts peat, one part coarse sand and one part perlite. A potting soil recipe for succulents is two parts soil, one part peat moss, one part coarse sand and one part perlite. Seedlings will grow well in a potting soil consisting of two parts peat moss, two parts compost and one part pre-wet vermiculite.