There are three basic elements of a soil mixture for indoor plants, each available in different forms, and each needed in varying proportions by different types of plants. The following is a basic recipe that should be varied depending on a plants requirements. For example, for plants that like soil "rich in humus," you would double the quantity of humus. For a "sandy soil mixture," double the amount of sand.
1 part soil - the "base" of most soil mixtures, often called "garden loam." Soil may be acid, neutral, or alkaline; clay-like or on the sandy side; high in humus content, average, or low. If soil is acid, add horticultural lime for plants that need it. If it is heavy with clay, add more sand; if sandy, add more humus. (Your County Agent will tell you how to have your soil tested for acidity or alkalinity, or you can test it with one of the available kits.)
1 part humus - to condition the soil, make it lighter and more porous, help hold moisture. Humus may be prepared and packaged, or scraped up from the forest floor. Peat moss, partially decayed leaf mold or compost, and manures - always well-rotted - are humus materials. Add less humus if soil is highly acid or already humus-rich.
1 part sand or substitute - to improve drainage, aerate soil, separate minute particles so roots have air to breathe. Use coarse builder's sand, not fine-grained or salty seashore types. Or substitute bird gravel, chicken grits, commercial brands of pelletized volcanic rock, coarse or fine vermiculite. Add extra sand to heavy clay soils.
Sift all ingredients through a screen with a mesh at least as small as a half inch, to remove stones and other undesirable foreign matter. Add fertilizers like bone meal or superphosphate according to each plant's needs.
To illustrate the "grain of salt" with which this recipe should be taken - most cacti and other succulents are potted like potting indoor plants in a mixture of three parts coarse sand or finest gravel with one part soil. Some growers add humus, some don't. Many add lime to neutralize acid. But the epiphyllums, orchid cacti, need a good proportion of humus.
Try to know your plants' needs, and suit the soil mixture to them. When plants are growing well, resist the temptation to experiment with some other soil mix, no matter who recommends it. It may be just the thing for your neighbor's plants, completely wrong for yours. When you do change soils, do it temperately and tentatively - try it on one or two pots before you take chances with more.