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What Is Potting Soil? (A Guide to Soil Substrates)

Rachel is a passionate plantsperson, YouTuber and author living in Ireland. She grows a wide range of hardy subtropical and tropical plants.

Learn about the different components in potting mix!

Learn about the different components in potting mix!

Compost, Mix and Soil, Oh My!

There is plenty of interchangeable terminology out there, and it also differs between countries. Here in Ireland and in the UK, we refer generally to the stuff that you buy in bags for your plants as 'compost'. We also use the term 'potting mix', but compost is the general term. In the US, compost is exclusively the product that comes from the breakdown of organic material typically on a compost heap. Here we call that stuff compost, too, just to confuse you.

In this article, you'll learn about the different substrates that you may need for potting or planting.

Potting Soil vs. Potting Mix

In the US, two widely used terms are 'potting soil' and 'potting mix', and there is much confusion about what the difference between them is and which to use.

  • Potting soil tends to be cheaper, unsterilized and contains garden soil, among other ingredients. It's heavy and it's best used in the garden.
  • Potting mix may contain a variety of ingredients, from fertilizer to perlite, and it'll be sterile. Potting mixes are used for potting up plants or seedlings.

Note: For those who are environmentally concerned, both potting soil and potting mix may contain peat. In any case, when buying any product, always read through the ingredients on the packaging to work out if it's suitable for your project and meets your requirements.

Basic Soil Components

MaterialDefinitionUses

Soil

Essentially, the uppermost layer of Earth's crust. Highly nutritious and composed of numerous materials.

Potting, planting and mulching.

Garden Compost

Decomposed plant and food waste.

Fertilising, improving soil and mulching.

Leaf Mould

Decomposed leaves.

Potting, improving soil and mulching.

Manure

Decomposed manure (preferably equine).

Fertilising and mulching.

Moss Peat

Partially decayed organic matter found in bogs.

Soil addition for acid-loving plants.

Sphagnum Moss

Still-living moss found on the surface of bogs.

Orchid potting and surface dressing.

Perlite

Expanded and heated volcanic glass.

Hydroponic growing, starting cuttings, increasing soil drainage.

Vermiculite

Naturally occurring mineral.

Increasing soil moisture and surface dressing.

Orchid Bark

Typically bark from fir trees.

Potting and increasing soil drainage.

Horticultural Sand

pH-neutral mix of crushed granite, quartz, sandstone, etc.

Increasing soil drainage.

Horticultural Grit

Larger pieces of crushed granite, quartz, sandstone, etc.

Increasing soil drainage and surface draining.

Learning about the different materials in soil can help you make your own custom blends.

Learning about the different materials in soil can help you make your own custom blends.

Let's expand on some of the basic ingredients that you can buy or create to make your own mixes. Some of the following are sterile. Others are not. Some are organic and some are inorganic.

Soil or Dirt

Soil or dirt is a heavy, highly nutritious ingredient. It can be an ingredient in potting, planting or as a garden mulch. It compacts easily, if not mixed with other ingredients.

Garden Compost

Garden compost is made from decomposed plant and food waste, and it's used as a fertiliser, soil improver or mulch, depending on how well rotted down it is.

Leaf Mould

Leaf mould is usually homemade from decomposed leaves. Well-rotted leaf mould can be used as an ingredient in potting mixes. Less well-rotted leaf mould can be used as a mulch on your flowerbeds or as a soil improver.

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Manure

Manure, specifically well-rotted horse manure, is an excellent fertilizer or mulch for your garden. Try to find a source that has been stored where weed seeds don't blow into it.

Moss Peat

Moss peat is partially decayed organic matter that's found in bogs and peatlands in solid form. It's naturally acidic and used for acid-loving plants like sarracenia, rhododendrons, et cetera. Peat is non-renewable, and when it's broken down, it releases carbon into the atmosphere.

Peat should be avoided, for all our sakes, but unfortunately you'll need to look hard to find a mix that doesn't include this crumbly substrate that has a long history of horticultural use.

Sphagnum Moss

Sphagnum moss is collected from a variety of still living mosses on the surface of bogs. It has amazing water retention properties and is used mostly in orchid potting and as a surface dressing on certain plants. It can be bought as a dried brick or crumbled into strands.

Perlite

Perlite is made from expanded and heated volcanic glass. It can be used for hydroponics or for starting cuttings. It has high permeability and low water retention and helps prevent soil compaction. It's invaluable in mixes where good drainage is required.

Vermiculite

Vermiculite is a naturally occurring mineral that's mined in various countries around the world. It can absorb three or four times its weight in water. It's useful for potting plants that do best in moist mixes. It's usually mixed with other ingredients and also an excellent top dressing for seedlings to keep mosses off.

Orchid Bark

Orchid bark is usually bark from fir trees, and it's used in orchid potting. It's composed of large pieces, but you can buy different grades. Besides orchids, this bark can be used for potting other plants that need good drainage, particularly epiphytes like the cheese plant or philodendron.

Horticultural Sand

Horticultural sand is made from substances such as crushed granite, quartz or sandstone. Mixing it with your other soils improves drainage. The difference between it and builders sand is that it's pH neutral.

Horticultural Grit

Horticultural grit is similar to horticultural sand, except the pieces are bigger. It's also used to improve drainage, and it's invaluable as a top dressing for cacti and succulents where you want to avoid splashback at the neck of the plant.

There are lots of other ways to spruce up your soil.

There are lots of other ways to spruce up your soil.

Other Materials

Finally, there are several other ingredients out there that can be used in your potting mixes, such as fertilizers, coir, bark chip, and less used ones like wool, sponge and akadama.

What About Premade Soil Mixes?

There are specialised, pre-packaged mixers out there, such as ones for potting orchids, succulents or cacti, African violets, streptocarpus, citrus trees, bonsai trees or acid-loving plants like rhododendrons. All of these contain combinations of different ingredients.

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© 2022 Rachel Darlington

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