“Potting soil,” for example, is very often sold as “garden soil,” and is often labeled “for in-ground use.” These products contain true dirt—mineral elements such as sand, clay, and loam. The best bagged soil products are formulated to mimic the components of the best natural soils found in the ground. True soil is generally not used for ordinary container gardening, but it is sometimes used to fill large raised beds, especially when the existing soil in a yard is of poor quality. Bagged soils are often mixed with compost to improve their nutritional value. “Potting mix” contains no dirt, but instead is a mixture of organic materials such as peat moss, along with elements aimed at improving drainage. These bagged products are sometimes labeled “soilless mix” or “soilless medium.” This is the standard product used for most container gardening, both indoors and outdoors. They sometimes contain slow-release fertilizers as a component.
They are indeed two different things and depending on what you need them for—potting or repotting container-grown plants, starting seeds, or adding soil to a garden bed, you need to make sure you get the right product for your purpose. Here are the differences between soil and potting mix as well as other useful tips about various potting mixes formulated for specific plants.
What Is Potting Mix vs. Potting Soil?
When purchased as bagged products, so-called “potting soil” (or “garden soil)” is much different than potting mix (which is soilless, and is often referred to as soilless potting mix). When shopping for a growing medium, read the label to know what you are buying. When the ingredients are not listed, the weight of the bag is usually a good indicator because a true soil will be much heavier than potting mix. Don’t be tempted by the colorful design on the bag. If the bag doesn’t list ingredients, don’t buy it.
Which One to Use?
For container gardening, both indoor and outdoor potted plants, as well as seed starting, use only soilless potting mixes. Their special combination of ingredients ensures that the mix retains moisture and won’t compact to allow enough air space for tender roots to grow. Also, in sterile soillesss potting mixes, plants or seeds are not exposed to disease pathogens or weed seeds. A soilless potting mix is a much more controlled growing medium than potting soil. When choosing a growing medium for non-container gardening and landscape use, use potting soil (often packaged as “garden soil”). It is also sometimes used to fill large raised beds or large elevated planters. Keep in mind though that potting soil, because it contains true mineral soil, can become 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 overall soil texture and drainage.
Aeration, Weight, and Drainage
Potting mix is the medium of choice for nearly all container gardening for three reasons: aeration, weight, and drainage.
Potting mix is a fluffier growing medium, which allows water and air to better penetrate to the roots in the confined area of a container. This fluffiness makes for better root aeration, which is essential to the health of plants. This fluffiness also improves drainage and prevents roots from stagnating in water, which can cause root rot. Finally, the fluffy, airy nature of potting mix also makes potted plants lighter in weight, which can be an important consideration if you are moving large potted plants around a patio or deck.
A true soil, by contrast, is typically much heavier and denser than potting mix. When used in containers, garden soil typically holds too much water to allow the roots to breathe and drain effectively.
Different Types of Potting Mixes
For most container-grown plants, you can use a standard all-purpose potting mix but some plants require exceptionally good drainage or a higher or lower pH. For these special conditions, you can purchase potting mixes formulated for specific types of plants.
Orchid potting mix: All-purpose potting mix retains too much water and too little air for orchids. Specialized orchid potting mix contains bark to drain well and create air flow that orchids require. Succulent and cactus potting mix: The sand in this type of 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 at a slightly alkaline range. African violet potting mix: A specially-formulated potting soil with pH adjusters such as dolomitic lime that meets the slightly acidic pH requirements of 6.0 to 6.5. Organic potting mix: A wide range of organic potting mixes are available. Look for the OMRI (Organic Materials Review Institute) label which certifies that everything that goes into the potting mix production is organic. If the mix contains fertilizer, only organic ingredients such as bone meal or bloodmeal are used. Moisture-control potting mix: Some potting mixes contain moisture control elements that absorb and retain more water than standard potting mixes due to the addition of spaghnum moss, coir, and wetting agents such as a non-toxic polymer.
