Compost is humus with microorganisms or organic matter that has been aged for some time. Potting soil is a sterile mix of organic materials such as peat moss, sand, sphagnum moss, perlite, and vermiculite. Compost is best used for outdoor plants. Potting soil is designed for garden beds and container plants.
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What is Potting Soil?
Also known as potting mix, potting soil is a mixture of inorganic and organic materials—it may or may not include sand and mineral soil. It’s a soilless growing medium often used for germinating seeds and growing plants in containers.
The organic materials that make up potting mix include rice hulls, sphagnum peat moss, coconut coir, compost, pines, and charcoal. These materials help in feeding the plants and retaining their moisture.
Its inorganic ingredients include sand, perlite, vermiculite, cinders, and pumice. These minerals help in drainage and provide aeration to the soil.
As such, potting soil is suitable for indoor potted plants such as cacti, orchids, succulents, and African violets, each having its own specific needs to be met to thrive.
There are different types of potting mixes depending on their intended use:
- All-purpose mix: It’s a mixture of perlite, peat moss, and vermiculite, making it ideal for most potted houseplants. Some brands opt for coconut coir instead of peat moss because it’s more environmentally friendly, while others add additives and wetting agents to enhance drainage.
- Orchid mix: Specifically mixed for epiphytic orchids and comes with ingredients such as perlite, vermiculite, and charcoal. These ingredients promote soil drainage and aeration while preventing waterlogging.
- African violet mix: Contains sand, perlite, and peat moss to provide proper water retention and drainage. It also has dolomite lime, a natural limestone that helps improve soil pH.
- Seed starting mix: Composed of coconut coir, perlite, vermiculite, and peat moss and is meant for starting seedlings and propagating.
- Cacti and succulent mix: Comes with ingredients like perlite or vermiculite formulated for cacti and succulents to provide extra moisture retention.
Extra Tip: Always check the ingredients and the plants compatible with the potting mix on the bag labels before purchase.
What Is Compost?
Compost is a soil amendment made from organic materials such as plant-based kitchen remains, decayed plant trimmings, and some animal manure. It’s a natural way to improve soil fertility, drainage, and aeration.
Compost contains nutrients such as phosphorus, potassium, calcium, iron, sulfur, nitrogen, and zinc, which promote the growth of healthy plants.
You can use compost as a top dress for your backyard, as a mulch over flower beds, or as a component in potting mixes.
Here are some types of compost you can use to improve your garden soil:
- Farmyard manure/animal compost: Made from a mixture of animal (herbivores only) dung and urine. This is the most nutrient-rich compost and is best used when your garden’s topsoil is nutrient deficient.
- Vermicompost/worm castings: This is a by-product of worms (red wiggler) used to turn kitchen scraps, leaves, coffee grounds, grass clippings, and other organic materials into a potent soil amendment.
- Plant-based: Also known as vegan compost, this compost is made of plant-based materials such as wood chips, grass clippings, coffee grounds, fruits and vegetable scraps, straws, and more.
- Municipal or commercial compost: This compost is made from organic waste collected in the community. It’s usually a mix of yard waste, food scraps, animal manure, grass clippings, and biosolids.
What’s the Difference Between Potting Soil and Compost?
The main difference between potting soil and compost soil is that potting soil is formulated as a growing medium for germinating seeds or container gardening.
Compost is a more advanced soil amendment made from decomposed organic materials.
| Key characteristics | Potting soil | Compost |
| Use | Growing medium | Soil amendment |
| PH levels | 5.8 and 6.2 | 6 and 8 |
| Texture | Loose and fluffy | Crumbly and smooth |
| Water absorption | Absorbs and retains water well | Less water absorbent than potting soil |
| Pathogens/contaminants | No pathogens and contaminants | Can include pathogens and contaminants |
| Nutrients | Fewer nutrients | More nutrients |
Other significant differences include:
1. pH levels
The pH levels of potting soils vary based on the ingredients used in the mixture. For instance, potting mixes made of peat moss have an acidic pH value of 3.0 to 4.5, while those with coconut air are slightly acidic (pH 5.8 and 6.5).
Compost is slightly acidic when still immature, but as it decomposes, it becomes neutralized. Typically, the pH level of finished compost varies between 6 and 8, while matured compost is neutral (pH 7.0).
2. Texture and Appearance
Potting soils are loose and fluffy (even when dry) when there’s good drainage and aeration. While compost has a crumbly and smooth texture—it becomes compacted when dry.
In terms of appearance, potting soils that contain vermiculite are brown-yellow, perlite white, and those with peat or coconut coir are dark brown.
On the other hand, compost is dark brown.
3. Water Absorption and Retention
Potting soils are suitable for absorbing and retaining water, especially mixed with sphagnum peat because they can hold up to 20 times their dry weight.
In contrast, compost doesn’t absorb or retain water well as peat because it’s hydrophobic.
4. Pathogens and Contaminants
Potting mixes are usually sterilized to eliminate weed seeds and pests, meaning they’re free of pathogens, fungi, and other harmful microorganisms.
Compost, on the other hand, can be a breeding ground for both harmful and good microbes.
For example, fresh manure may have pathogens such as Salmonella and E.coli, while commercial compost may contain contaminants such as herbicides and pesticides.
5. Nutrients
Since potting soil is usually sterilized, it doesn’t contain all the nutrients needed for plant growth.
On the other hand, compost is a rich source of essential nutrients such as nitrogen, phosphorus, potassium, and magnesium.
When Should You Use Potting Soil?
Potting soil is ideal for seed germination and propagation because it doesn’t contain harmful bacteria that can destroy young plants.
Its good drainage properties make it the best choice for container plants and succulents.
This is also an excellent option for indoor plants because it’s sterilized and therefore can’t attract diseases or pests.
When Should You Use Compost?
You can use compost to supplement potting soil with essential nutrients or add beneficial microbes to your garden’s topsoil.
Compost is suitable for fruits, flowers, herbs, and vegetable gardens.
Does Potting Soil Contain Compost?
No. Potting soil doesn’t contain compost.
However, you can add compost to your potting mix to supplement it with essential nutrients.
The Verdict: Potting Soil or Compost?
Even though potting soil is often mistaken for compost, they are not the same. However, each option has unique features, advantages, and uses.
The best option will depend on your gardening needs—what you’re growing, the location, and garden soil condition.
For instance, compost is great for adding nutrients and shaping the soil, making it suitable for outdoor plants. It’s also a cheaper means because you can make it at home using organic materials such as food scraps, yard waste, grass clippings, leaves, and animal manure.
Potting soil, on the other hand, is designed for garden beds and container plants. It has a higher pH level that helps prevent plant roots from rotting and spreading diseases and pests.