The potting soil can contain several ingredients but must be well drained. Add organic matter such as peat moss for plants requiring more soil moisture. Packaged potting soils often contain much organic matter and may require additional perlite, sand or vermiculite to improve drainage.
Each indoor gardener needs to experiment to find a soil mix that works best for them. People who tend overwater may want to add about 1/2 to 1/3 perlite or vermiculite to packaged potting soil.
Potting soil high in organic matter may shrink if it dries out. Then, when the plant is watered, the water runs off the soil and down the space between the pot and soil, without ever soaking into the soil. To solve the problem, set the plant in a container of water. Once the soil is wet, take the plant out of the water and let it drain. Then firm down the soil around the rim of the pot to close the gap.