The Breakdown of Soil

Ever wonder what the difference is between all the different kinds of soil? This guide will give you a better understanding of the different types of soil, their ingredients, and each ingredient’s purpose in the medium.

Potting Soil vs. Garden Soil

There are two major types of soil mediums or mixes available on the market, and it is important not to interchange them, though the names are very straight-forward: potting soil and garden soil. As you may have guessed, potting soil is meant to be used on its own in pots or containers, while garden soil is meant to be used as an addition to your existing soil in your garden or lawn. While both may contain similar ingredients, and even share some exact amendments, they are formulated for two very different purposes. Within the two types, there are numerous brands, blends, and ratios for different plant species or uses. It is also important to note that any good quality brand will normally heat-treat both potting and garden soil before bagging it to kill any present weeds or harmful microbes.

Potting soil is typically a mixture of peat moss, perlite, and humus, and any additives necessary for the purpose of that specific blend. It is usually pH balanced, contains an amendment for water retention, and could contain a wetting agent to help saturate the soil. It also includes a small quantity of fertilizer to provide some nutrient content for your plants. As potting soil has been refined and normally has additional ingredients, it is almost always more expensive than garden soil.

Garden soil is generally a high quality top soil, blended with amendments, such as peat moss, sand, and compost, to improve the structure of your existing soil. It is usually pH balanced, and contains some nutrient value for your plants. The primary garden soil that is sold in Canada is referred to as triple mix.

Main Components

Both potting soil and garden soil typically have multiple ingredients that are combined to make up the medium. All of these ingredients contribute to the medium’s effectiveness in a different way, and by working together, they promote a healthy root system and a healthy plant. Below are some of the common components that you will encounter and their intended purpose.

Humus or Black Earth: A dark, organic material that is left after plant material and animal remains decay

Sphagnum Moss: Sphagnum moss is a plant that grows on top of bogs, through Canada, New Zealand, Ireland, and more. It is harvested, dried and sterilized and can be sold both individually or as part of a mixed medium. It is used either as an orchid or seed-starting medium on its own, or for water retention when added to potting soil.

Peat Moss: Peat moss comes from dead and decaying sphagnum moss that is found at the bottom of bogs, and can contain other organic material mixed in. It can be soil individually in compressed bales or it can be an amendment in many potting and garden soils. It does not contain any nutrient content, so it is normally accompanied by a fertilizer of sorts. Peat moss has a high water retention rate and does not compact, providing good aeration for the roots.

Coconut Husk Fibre / Coco Coir: Shredded coconut husk that is also used for water retention and aeration, so it can be an alternative to peat moss.

Perlite: A volcanic glass that has been heated until it ‘pops’ and expands, producing a lightweight material called perlite. Perlite can help retain moisture in soil, but it is much better suited for increasing drainage and reducing compaction in your soil. It does not retain as much moisture as vermiculite, and is therefore best used as an amendment to soils for lower-water plants.

Vermiculite: An aluminum-iron-magnesium silicate that is also heated until it expands. Once expanded, vermiculite can absorb up to 4 times its weight in water. This makes it an ideal soil amendment for those looking to significantly increase moisture retention in their potting mix. It retains more moisture than perlite and therefore best used for moisture-loving plants.

Compost: Decayed organic material, ranging from seaweed and kelp, to barnyard manure, to woodland bark. It is added to soils to improve the soil structure while increasing the amount of nutrients in the soil for the plant to uptake.

Dolomitic Limestone: A combination of calcium carbonate and magnesium carbonate. It is used to reduce soil acidity and provides a source of magnesium and calcium for the plant. It is often paired with peat moss to balance the pH of the medium.

Sand: Used to improve soil drainage.
Bark: Bark can be found both individually as an orchid-growing medium, or as an additive in a soil mix. It helps improve drainage and aeration by loosening up the soil.

Worm Castings: The waste that earthworms produce after they have digested the organic matter in soil. It is used as an organic fertilizer as it contains helpful nutrients and enzymes for plants, as well as improving soil structure.
Wetting Agent: A chemical compound that reduces the surface tension of water allowing it to penetrate and absorb into the soil easier and more thoroughly.

Types of Soil

Below is a list of common soils you will encounter and their primary uses.

Triple Mix: A combination of peat moss, compost and top soil. A light soil mixture that is good for top dressing lawns or adding to garden beds to improve the existing soils structure.

Black Earth: Mostly soil but will sometimes contain manure or other additives. Generally used for dressing existing garden beds to help amend soil and improve the appearance of garden beds.

Top Soil: A mixture of sand and clay soil. A good topsoil will be screened to remove any large pieces of debris. Used for filling holes, preparing the ground to lay sod, or leveling and grading your landscape.

Tropical Potting Mix: A combination of peat moss, black earth, a water-retention agent like perlite, vermiculite or coco coir, and other additives. Formulated for tropical plants in containers that prefer moist soil.

Cactus and Succulent Potting Mix: A combination of peat moss, black earth, sand, a water-retention agent like perlite, and other additives. It is a fast-draining soil medium for plants in containers that need to dry out quickly.

Orchid Mix: These can be anything from bark, sphagnum moss, or a mixture of both with other additives. Generally most orchid mixes will be very fast draining and resist compaction so roots are able to get air.