If you tried to grow container plants using regular soil, you would find that the plants do not grow well, and may actually die. This is because soil contains microbes that can grow until they are out of proportion to the soil content in a container and can harm the plants placed in the soil. Instead, most container gardeners use a soil-less potting mix to grow plants. Potting mixes contain organic materials that nurture plants and provide a substrate for root development.
Learn about the natural soil conditions of the plant that you plan to grow in containers. Select a potting soil that mimics the natural soil conditions that the plant thrives in. While most plants thrive in a general purpose potting mix, orchids grow well in a bark-based mix that mimics the bark of trees that the plants grow in while cactus grow well in perlite or vermiculite that resembles sandy desert soils.
Examine the bag of potting soil before you purchase it. Most soil bags differentiate between potting mixes that contain no loam, such as orchid mix, and potting soils that contain loam to mimic real soil. The label will also list the ingredient ratios of contents including organic materials, volcanic or rock material for drainage, fertilizer and moisture retaining crystals as well as whether the soil has been sterilized. Never purchase soil that has not been sterilized to remove bacteria that can harm a plant. Select soil that contains a good balance of loamy contents such as compost, peat moss and aerating contents such as perlite. You may also wish to purchase a potting soil that contains nutrient-rich fertilizer or manure. The presence of moisture retaining crystals will help you avoid watering frequently.
Scan the bag to determine the weight of the soil. Shade or hanging basket plantings will use a lightweight potting soil. Full sun plantings require a medium weight soil. Avoid heavyweight potting soils. These soils will retain too much moisture, and can promote root rot in plants.
Feel the weight of the bag and look to see if it is dripping with water. Avoid purchasing waterlogged bags of potting soil. Potting soil that has become waterlogged in the bag may become compacted, and may provide a breeding ground for bacteria.