Many people have used potting soil and compost in growing plants, but most people use them interchangeably without taking advantage of their characteristics.
What are the differences between potting soil and compost?
Compost is decomposed organic matter or humus with micro-organisms that make the nutrients in soil more accessible for immediate uptake over time. Potting mixes contain both organic and inorganic substances, but they are sterile (without pathogens, fungi, and insect eggs) making them suitable for starting seeds, growing seedlings and houseplants.
In this article, we will look at the differences between these two popular growing substrates and explain how to select the right one for your next gardening project.
1.1 Types of potting soil
Potting soil is often referred to as “potting mix”, both of which may or may not contain soil.
Potting soil is manufactured and sold commercially to provide a sterile growing medium for seed germination as well as for growing potted plants indoors, e.g. succulents, cacti, orchids, African violets, with different needs for moisture and soil aeration.
The individual ingredients can be organic, like coconut coir, sphagnum peat moss, rice hulls, pine and fir bark chips, charcoal, and compost. They can also be inorganic materials like perlite, vermiculite, pumice, sand, or cinders. They are available in different ratios to suit different types of plants.
The major potting mixes and their ingredients are:
- All-purpose mix: The all-purpose potting mix is mostly a blend of peat moss, perlite, and/or vermiculite which are suitable for a variety of houseplants. For some brands, coconut coir is used instead of peat for environmental reasons. Some manufacturers also include additives like organic fertilizers and wetting agents to help absorb water more easily.
- Cacti and succulent mix: Cacti and succulent mixes contain more perlite or vermiculite than regular mixes to allow the soil to dry out quickly, as preferred by succulents that are drought-resistant.
- Orchid mix: Orchid mixes for epiphytic orchids do not contain soil, but bark chips with perlite, vermiculite, or charcoal for drainage and aeration as they grow on trees instead of in the ground.
- African violet mix: This mix contains peat moss, perlite, and some sand. It also contains dolomite lime to give the mix a slightly acidic pH as required by African violets.
- Seed starting mix: This is a light and sterile mix used mainly for propagation, consisted of peat moss or coconut coir, perlite, and vermiculite.
- Peat-free potting mix: As you can tell from the name, these mixes don’t contain peat. Coconut coir is used in place of peat as it has similar water-holding abilities, and it’s a renewable resource, unlike peat moss.
1.2 Types of compost
Compost is purely organic matter that is well decomposed. There are many types:
- Animal compost: This is made of animal manure, primarily horses, cows and other herbivores. Manures from non-herbivores aren’t often used for composting as they carry a lot of harmful bacteria.
- Plant-based or vegan compost: This compost is made using only plant matter like leaves, grass clippings, wood shavings, straw, wilted plants, coffee grounds, fruit and vegetable scraps.
- Vermicompost: Also called worm castings, this is essentially worm poop. Certain worms, particularly Red wiggler worms, are fed with kitchen scraps, grass clippings, coffee grounds, and other organic materials. The worms eat the organic materials and produce excrement rich in plant nutrients and can be used as a soil amendment in the garden.
- Municipal compost: This type of compost is made with organic wastes collected from entire communities. This waste includes food scraps, grass clippings, biosolids, animal manure, and even road kills. The resulting compost is neither plant-based nor animal manure but a mixture of both. One downside to municipal or commercial compost is that you’re never sure of what went into it – there may be some inorganic fertilizers and herbicides from grass clippings.
Finished compost is dark, almost black, as the organic matter has decomposed to the point that it’s no longer visible. It has a smooth, crumbly texture. It may become compacted when it dries out.
The appearance of potting mixes varies depending on the type. Mixes containing peat or coconut coir have a dark brown color speckled with the white from perlite and black, yellow or brown, from the vermiculite.
A good potting mix stays fluffy with good soil aeration and drainage even when it dries out. It crumbles to tiny fibers, but you will also feel the grittiness from the perlite, pumice, or vermiculite.
Compost starts off being slightly acidic, as acidic conditions help break down the organic matter and encourage the growth of fungi. But as the compost continues decomposing, it becomes less acidic until it is neutralized. Generally, finished compost has a pH between 6 and 8. In most cases, matured compost has a neutral (7.0) pH.
Potting mixes have different pH levels depending on the components of the mixture.
Potting mixes that contain peat are usually acidic, as sphagnum peat moss has an acidic pH between 3.0 and 4.5.
When coconut coir is used instead of peat, the mix tends to be slightly acidic and neutral, as coconut coir has a pH of 5.8 and 6.5.
Potting mixes with peat have good water absorption and retention, as sphagnum peat can hold up to 20 times its weight in water. Some may come with wetting agents to help water to penetrate the soil.
Compost also has good water retention but not as much as peat. However, it becomes hydrophobic, making it difficult for water to penetrate when it dries out.
Potting mixes have low nutrient content because organic components like peat moss, coir, and bark have little to no nutrients at all. Some manufacturers include slow-release starter fertilizer to make up for the lack of nutrients. It is also sterilized to contain no micro-organisms.
In contrast, compost is rich in humus, or well-decomposed organic matter, which contains the major plant nutrients – Nitrogen, potassium, phosphorus, and other macro and micronutrients like calcium, sodium, zinc, magnesium, and iron.
The following table shows the nutrient contents in farm compost and vermicompost:
|Nutrient||Farmyard compost (%)||Vermicompost (%)|
|Iron||146.5 ppm||175 ppm|
|Copper||2.8 ppm||5 ppm|
|Zinc||14.5 ppm||24.5 ppm|
|Manganese||69 ppm||96.5 ppm|
6. Pathogens and Contaminants
A good potting mix is sterilized when you get them, i.e. free of insect eggs, pathogens and fungi.
Compost, on the other hand, can contain both the good and the bad bacteria, fungi and weed seeds. It may even contain herbicides, pesticides, and inorganic fertilizers, e.g. in municipal compost.
Potting mixes are better for seeds and propagation. That’s because they’re free from pathogens that can kill the young plants, they can keep the soil moist, and they have good drainage to prevent wet and soggy conditions. Even the lack of nutrients can be easily remedied with the right fertilizer.
Potting mixes are better the compost because it is sterile without pathogens, diseases, and gnat eggs which can be difficult to contain indoors.
Use compost soil for outdoor gardens. It’s a less expensive alternative than filling up a garden with potting mix. Compost contains a variety of micro-organisms which would help the organic matter in the soil to decompose and slowly release plant nutrients over time.
It’s best to use compost for vegetables. It is not only cheaper, but compost also contains a variety of microbial activities which can help break down the plant nutrients in the soil over time and helps the proper growth of organic vegetables.
You can plant in only potting mixes because they are formulated to be substitutes for soil. As long as you’re using the right mix for your plant, you should get good results.
But you will need to fertilize the mix at intervals, as potting mixes generally don’t have nutrients.
9. Can I Plant in Only Compost?
While it may seem like a good idea at first, it’s not advised to plant in compost. Compost is a soil amendment; it is not soil or a suitable alternative like potting mixes.
First, compost is light, and can’t support the root structure of plants. Furthermore, compost drains easily, leaving plants needy for water.
There are a ton of products available on the market. And you can’t simply choose one at random, as that will likely lead to problems down the line.
Here are some tips to help you select a good potting mix
- Check the label for any mention of chemical contaminants, heavy metals like lead and mercury, or pesticides. A good potting mix shouldn’t have any of these.
- Check for the pH of the mix. This is crucial for acid-loving plants like berries, tomatoes, azaleas, etc.
- If possible, select a peat-free mix, as peat isn’t a renewable resource
There’s no harm in adding compost to the potting mix. In fact, some potting mixes contain compost. Adding compost to your mix will provide some nutrients to the plants.
Add 10 to 20 percent of compost. You don’t need to mix it with the mix, just spread it on top, and worms, water, and microorganisms will do the mixing for you.
It is important to differentiate between potting mix and compost because they are best suited for different gardening purposes.
Potting mixes are a sterile growing substrate for plants, particularly indoor plants. Potting mixes generally do not have nutrients, but they are perfect for propagation and starting seeds, as they are sterile and do not contain any pathogens that may affect young plants.
On the other hand, compost is a soil amendment added to soil to improve soil fertility, aeration, and water retention. Compost is affordable, and it contains nutrients which makes it an excellent choice for outdoor gardens and growing organic vegetables.
Garden Design Magazine. (2021, December 23). Potting Soil 101: How to Choose the Right Potting Mix for Your Plants.
Hannan, J. (2016). Soil pH in The Home Garden | Horticulture and Home Pest News. Iowa State University.
Monitoring Compost pH – Cornell Composting. (n.d.). Cornell University.
Punjab State Council for Science and Technology (2010). Vermiculture / Vermicompost.
Smith, E. (2019, July 1). Suggested Blueberry Fertilization Timings and Rates. University of Georgia.