Picking up a bag of potting soil that fails to produce the results you want is a frustrating experience that can waste weeks of growing time. The best potting soils combine quality materials to create a growing medium that stores more water than regular garden soil without saturating the roots of your plants. High quality potting soils are designed to resist compaction and store water and nutrients.
Soil and Structure
Regular garden soil used in containers tends to become compacted over time due to the frequent irrigation regimen that most potted plants require. Potting soils use a variety of materials to create and maintain spaces in the soil known as pores that allow water and air to reach the roots of your plants. Some materials that are added to potting soils to improve the drainage and aeration of potting mixes include bark, perlite, gravel and coarse sand. Potting soils with large chunks of bark or gravel are often used to support orchids, while mixtures designed for cacti and other succulents often contain a larger proportion of sand.
Potting soils use a variety of materials to store water and nutrients. Some common additions include sphagnum or peat moss, vermiculite, compost, water retention crystals and soil. Potting soils made with compost use decomposed plant matter to provide nutrients and retain water effectively. Peat moss is another amendment with a low pH that is often included in potting soil for its ability to store water and nutrients.
Some potting soils come with fertilizer added; however, most potting soils hold little nutrients of their own. Check the label of your potting soil to determine if it contains fertilizer, and how long it can supply nutrients to your plants. Plants kept in potting soil require regular fertilization. Using a slow-release fertilizer reduces the number of times you'll have to fertilize your plants and allows for more steady growth.
The content and quality of potting soil is not regulated. The contents of commercial products often vary in their physical and chemical characteristics. Many low quality potting soils have high levels of soluble salt or a low pH that can be detrimental to your plants. Heavily irrigating your potting soil several times before you use it helps wash salt from the soil and reduces the risk of damage to your plants. Low quality potting soils often use Styrofoam instead of perlite. You can tell if your potting soil contains Styrofoam because it tends to float to the surface of water more easily than perlite, and it becomes crushed and compacted over time.