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What is the difference between Compost vs Potting Soil?
You have probably heard and will most likely still hear when it comes to gardening that to get beautiful and healthy gardens, you need quality soil. A large part of gardening success is ensuring you use quality soil, but how do you know what to buy and ensure what you get is quality?
Whilst a lot of gardening involves learning and going through the experiences, knowing and learning about soil and amendments can simplify things. Let’s see the differences between compost and potting soil, and how they are best used to achieve the gardening results you desire.
What are compost and potting soil?
What is Compost?
What are the differences between Compost and Potting Soil
Compost is a greatly beneficial amendment that is useful in improving the soil’s texture and increasing its nutrients. It is made from natural products like organic food waste, decayed vegetation and animal manure. Then, they are mixed with soil or finished compost and added to fertiliser.
Also, compost is usually added to soil in clay soils to improve aeration and drainage, and in sandy and rocky soils to retain water and nutrients. By feeding plants and soil microorganisms, a healthy growing foundation is created for gardening. Composts are also usually filled with nutrients which contribute to and improve soil structure and increase its fertility.
What is Potting soil?
Potting soil is, however, designed as a healthy and unpolluted habitat to grow seeds or to support plants grown in pots by providing well-textured soil. The composition of potting soil usually determines how many nutrients it contains; however, they are generally fewer nutrients. It provides a safe, hygienic environment for plants growing in containers.
Major Differences between Compost and Potting Soil
Understanding their differences will guide you in knowing the right material and application to apply in a gardening task.
Origin and Composition of compost vs potting soil
Compost is composed primarily of organic matter that has decomposed well. It however has different types based on what the organic matter is made up of. Examples of the different compost types are:
- Multi Purpose Compost – Standard compost found in garden centres. A mix of organic matter and often Peat (which you should avoid). Often containing slope release fertiliser
- Animal compost: It is made from animal manure like cows, horses and other herbivores. They are not used often because they harbour a lot of harmful bacteria.
- Plant-based compost: This is the most commonly used compost and it is made from decaying plants like leaves, fruit and vegetable wastes etc.
- Municipal compost: This is a mixture of both animal and plant-based compost and it is made from natural wastes gathered from different areas, including food waste, grass, biosolids, animal manure etc. Some people love it, and some people would never use it
- Mushroom compost: contains more nitrogen than normal compost, and is made from the growing material from commercial mushroom growers. A popular choice these days if you can afford the extra.
- READ MORE: What is the difference between John Innes number 2 and 3 composts and why does it matter?
Potting soil however may or may not contain soil and are usually produced and sold to supply a hygienic environment for growing seeds and indoor plants. Their ingredients can be natural and inorganic for example coconut coir, charcoal, compost, vermiculite, pumice, sand etc, and are put in different quantities to suit the planting need. Some examples of potting soils are:
- All-purpose mix: A combination of peat moss, perlite, and/or vermiculite which works well for a large number of indoor plants.
- African violet mix: It contains peat moss, perlite, some sand and dolomite lime to give it a slightly acidic pH as required by African violets.
- Seed starting mix: This is a light and hygienic mix used mostly for propagation, and consists of peat moss or coconut coir, perlite, and vermiculite.
Differences in Appearance and Texture between Compost and Potting soil
A completed compost has a dark, almost black colour because the organic waste has decayed to a point where it is no longer visible. It is usually smooth and crumbly in texture but may become compressed when dry. However, potting soil varies in looks largely based on the type.
Mixes that contain peat or coconut coir have a dark brown colour speckled with the white from perlite and black, yellow or brown, from the vermiculite. A good potting mix will remain fluffy with good soil ventilation and drainage even when dry. When it crumbles to tiny fibres, you will feel the gritty texture from the perlite, pumice, or vermiculite.
Differences in pH between Compost and Potting soil
Compost starts slightly acidic, as it helps the organic waste break down and encourages the growth of fungi. As the compost continues decaying, it becomes less acidic until it is fully neutralised. Generally, a finished compost has a pH between 6 and 8, and in most cases, a matured compost has a neutral (7.0) pH.
Potting soils as usual have different pH levels based on the elements of the mixture. Potting mixes consisting of peat are usually acidic because sphagnum peat moss has an acidic pH between 3.0 and 4.5. When coconut coir is used in place of peat, the mix tends to be slightly acidic and neutral, as coconut coir has a pH of 5.8 and 6.5.
Differences in Water Absorption and Retention between Compost and Potting soil
Compost has good water retention but not as much as potting soils because of the peat in the mix. It can however start repelling water making it difficult for it to penetrate when it dries out. Potting soils with peat on the other hand have high water absorption and retention rate because sphagnum peat can hold up to 20 times its weight in water. Some potting soils may also come with wetting agents to help water to penetrate the soil.
Differences in Nutrients between Compost and Potting soil
Composts are rich in well-decayed natural waste and contain the major plant nutrients like Nitrogen, potassium, phosphorus, and other macro and micronutrients. Potting soils, however, have low nutrient content because the natural elements like peat moss, coir, and bark have little or no nutrients, and are also sterilised to contain no micro-organisms. Some manufacturers add fertiliser to substitute for the absence of nutrients.
