Potting soil and potting mix are terms that can confuse beginner gardeners. Both potting soil and potting mix are growing media, i.e., materials in which plants can develop their roots. Both terms usually refer to media that are meant to be used in containers, not in raised beds or in the ground. From there, providing more precise definitions gets tricky.
What Is a Potting Medium?
First of all, don’t be confused by the terms media vs. medium. Media is simply a plural form of medium. A medium, in the words of the Cambridge Dictionary, is “a substance that something grows in, lives in, or moves through.” A growing medium is any material that’s used for growing plants, such as soil or compost. A potting medium is a growing medium that’s used for growing plants in containers, such as pots or trays.
Potting Mixes and Marketing
“Potting mix” and “potting soil” aren’t scientific terms. They’re names used by manufacturers to label their products or used by gardeners to describe their homemade mixtures.
You may notice that the use of these terms varies, depending on where you’re located, who you’re talking to, or which product label you’re reading. Both potting mix and potting soil most often refer to growing media with ingredient lists that do not include any “true soil.” Or, potting soil may also refer to a soil-based medium. Confused yet?
In this guide, we’ll cover the components and uses of potting mixes and potting soils to help you decide on the best solution for your plant friends. Keep in mind that these terms are used inconsistently. As long as you understand which individual and blended potting ingredients are helpful for plants, you won’t get tripped up deciding between a bag of potting soil vs potting mix.
What is Potting Mix?
Potting mix is a combination of several non-soil substances used for helping plants grow in containers. Potting mixes typically contain a blend of inorganic (from non-living sources) and organic (from living sources) components. Inorganic components help with aeration and adjust a potting mix’s water retention and drainage properties. Organic components help a potting mix efficiently hold water and air while attracting helpful microorganisms.
Inorganic components may include:
- Perlite or vermiculite: porous, flaky mineral materials
- Pumice or cinder: gravelly volcanic rock fragments
- Horticultural sand: granular mineral material
Organic components may include:
- Peat moss: fibrous substance formed by decomposed organic matter in peat bogs; helps growing media retain moisture and improves pH levels for acid-loving plants
- Compost: decomposed organic material; may be made from forestry waste, leaves, or food waste
- Earthworm castings: natural fertilizer from earthworm excrement; improves water retention, aeration, and soil enrichment
- Coconut coir: fibrous waste product from coconut husks; used as a sustainable alternative to peat moss
- Tree bark: often sourced from pine trees; stabilizes soil and improves water retention
- Bat guano: effective fertilizer and soil builder, rich in nitrogen, phosphorous, and potassium
- Organic or chemical fertilizer: natural or artificial substance that provides nutrients, i.e. “plant food,” to help plants grow
The various inorganic and organic ingredients in a potting mix combine to form a medium that’s lighter and fluffier than true soil. Potting mix enables aeration, drainage, and water retention within the confined space of a container. Potting mix products are also sterile, unlike most soil products. That means they’re free of pathogens, seeds, and pests.
What is Potting Soil?
First, let’s define soil. Soil is a mixture of extremely small rock and mineral particles, water, air, and dead and living organisms (organic matter). Fertile soil is full of living things, including fungi, bacteria, and nematodes.
The term potting soil is often used interchangeably with potting mix. But the use of the word “soil” doesn’t necessarily mean the mix contains actual soil. In fact, part of the reason for using a commercial potting mix is that the ingredients are usually sterilized to destroy organisms that could potentially damage your potted plant. So, even if a product is called “potting soil”, that doesn’t mean it contains what would be considered true “soil”.
True soil-based products are designed to resemble an exceptionally healthy soil arrangement that could occur naturally in the ground. These products include soil along with ingredients like compost, peat moss, and fertilizer. Soil-based mixes are best used for growing plants in the ground or in raised beds.
Soil taken from the ground or soil-based mixes are usually not appropriate for growing plants in containers. Soil is typically dense and heavy, lacking the aeration and drainage properties that are crucial for encouraging healthy root growth in closed containers.
Choosing between Potting Soil vs Potting Mix
If you’re buying a bagged growing media, read the product information to see if it’s recommended “for in-ground use.” If it is, then it may be a soil-based product that isn’t meant for pots or containers.
In general, potted plants thrive in light and fluffy media. Ideal potting mixes typically contain some organic material (peat moss, coconut coir, bark, compost) along with perlite or vermiculite. The goal is to provide your plants with a growing medium that keeps roots aerated and provides a balance of drainage and water retention.
Of course, every plant is different. Before mixing or purchasing a medium, always research the ideal growing conditions for each individual plant that you’re starting or repotting. When possible, choose a potting mix specifically designed for your plant. For example, customized mixes can be found for cacti and succulents, orchids, flowering plants, and seed starting.
Interested in taking your soil knowledge a step further? Next, read our guide on garden soil and mulch to learn how to develop and improve your outdoor garden soil.
