Shopping for a growing medium to use for potted plants can be a confusing experience as the terms “potting soil” and “potting mix” are often used interchangeably. They are indeed two different things and depending on what you need them for—potting or repotting container plants, starting seeds, adding soil to a garden bed, or raised beds—you can use either one, or you need to make sure you get the right product for your purpose.
Here are the differences between potting soil and potting mix, as well as other useful tips about various potting mixes sold for indoor or outdoor use.
What Are the Ingredients in Potting Soil and Potting Mix?
Potting soil and potting mix (which is soilless, often referred to as soilless potting mix) have some key differences. When shopping for soil, read the label to find out what you are buying. When the ingredients are not listed, the weight of the bag is a usually good indicator because the soil contained in potting mix makes it much heavier than potting mix. Don’t fall for the colorful design on the bag if there is no label spelling out all the contents—don’t buy it.
|Potting soil||Potting mix|
|Contains garden soil||Soilless, also called soilless mixes or soilless media|
|Not sterile—may contain pathogens such as fungi or diseases and weed seeds||Sterile|
|Contains minerals and organic matter||Contains organic and inorganic components to improve aeration and drainage: peat moss, spaghnum moss, aged bark (pine bark), coir (from coconut husks), pumice, styrofoam, perlite, vermiculite|
|Contains compost||Contains slow-release starter fertilizer or continuous fertilizer|
|Heavy||Lightweight and fluffy|
What to Use for Which Type of Gardening
For container gardening, both indoor and outdoor potted plants, as well as seed starting, use only potting mixes. Their special combination of ingredients ensures that the mix retains moisture and won’t compact so there is enough air space for tender roots to grow. Also, in sterile potting mixes, plants or seeds are not exposed to any disease pathogens or weed seeds. A potting mix is a much more controlled growing medium than potting soil.
For non-container gardening and landscape use, potting soil is usually fine. Keep in mind though that potting soil, because it contains soil and sometimes sand, can get compacted, dense, and water-soaked. While potting soil might contain a good amount of nutrients from the compost it contains, you will still have to add other amendments to improve the overall soil texture and drainage.
Take the information on the bag about how many months the fertilizer will last with a grain of salt. The frequent watering of container plants flushes out the fertilizer and some plants, especially edibles that are harvested and regrow, are heavier feeders than others. Just because a mix has added fertilizer, it does not guarantee that it will keep the plant healthy and vigorous during the entire growing season. One of the most common container gardening mistakes is underfeeding your plants.
Different Types of Potting Mixes
For most potted plants, you can use a standard all-purpose potting mix but there are some plants that require exceptionally good drainage or a higher pH. For these you can purchase special potting mixes:
- Orchid potting mix: All-purpose potting mix holds too much water and too little air for orchids to grow. Special orchid potting is coarser to create the necessary air pockets.
- Succulent and cactus potting mix: The addition of sand in this potting mix ensures the fast drainage that is key for growing succulents and cacti. A variation of this is cacti, palm, and citrus potting mix, which also contains recycled forest products and pH adjusters such as oyster shell lime to keep the potting mix in the required slightly alkaline range of 6.0 to 7.3.
- African violet potting mix: A special potting soil with pH adjusters such as dolomitic lime that meets the slightly acidic pH requirements of African violets of 6.0 to 6.5.
- Organic potting mix: In addition, there is a wide range of organic potting mixes available. Look for the OMRI (Organic Materials Review Institute) label which certifies that everything contained along the chain that goes into the potting mix production is organic. For fertilizer, these products contain organic ingredients such as bonemeal or bloodmeal.
- Moisture-control potting mix: There are potting mixes with special moisture control that absorb more water than standard potting mixes due to the addition of spaghnum moss, coir, and wetting agents such as a non-toxic polymer.
Amending Potting Soil
The peat moss contained in potting soil decomposes quickly. To successfully grow plants long-term in potting soil, amend it by adding perlite. There are other tricks to amending potting soil to improve it and the wellbeing of your plants, including annual repotting and flushing the soil monthly.