What is Potting Soil made from?

I stood in the gardening section of a big store recently, and was confused by choice. I only needed some soil in which to re-pot some plants that had outgrown their containers. I really didn’t know which one to pick, so went for the cheapest - it looked like it had some other important other stuff in it, and also because it had ‘eco’ printed on the bag. That must be good, mustn’t it? Thumbs up so far. Saving me money and saving the planet. When opened, it felt moist, and had pieces of wood in it, which I hadn’t expected.

When faced with all of the different soil mixes available, deciding on garden soil vs potting soil can be difficult.

Some are for potting up orchids, or African violets, or cacti, succulents, and more. To work out which one might be best for your gardening project, apparently, it’s important to understand which ingredients are typically found in both garden soil and potting soil, then you can fill your garden or container accordingly so the plants, seeds or seedlings get the best start.

So as a general rule, it seems garden soil is used in outdoor raised beds or mixed into traditional garden beds. Potting soils and mixes are often used when potting up outdoor container arrangements, potting (or re-potting) houseplants, and for seed-starting and plant propagation. They are generally lightweight and sterile, whereas garden soils are usually heavier and potentially teeming with life.

Why garden soil and potting soil are not interchangeable

Garden soil and potting soil are not actually the same thing, although you may see them referred to interchangeably. They each have different qualities which make them better suited for different purposes.

What is garden soil?

Garden soil is just topsoil to which organic materials, like compost, worm castings, and aged manure have been added. On its own, topsoil is used in landscaping projects like filling in low spots or establishing new lawns. It contains organic matter and, depending on its source, varying amounts of different particle sizes, including silt, sand, and clay. It is rich in organic matter, is heavier than potting mixes, and contains a range of beneficial microorganisms, but it may contain some weed seeds and plant pathogens, depending on its origin. It provides good support for roots and top-heavy plants, and is good at retaining moisture. Garden soil contains a whole network of tiny, living creatures—soil microbes, like beneficial fungi and bacteria. As these microorganisms naturally break down organic matter in the soil, they increase the bioavailability of nutrients, helping plants to thrive.

What is potting soil?

Potting soil is a growing medium often used for seed-starting and container gardening. Potting soils may contain a base of garden soil, aged compost, or composted wood along with non-soil additives. such as perlite, vermiculite, and peat moss or coconut coir. Some of these extra ingredients add structure and support for plant roots. Others help to retain moisture or provide room for oxygen around developing plant roots. It is sterile (contains no weed seeds or plant pathogens), but provides no nutrients, unless fertilizer has been added to the mix. Bagged potting soil is formulated to retain moisture and promote aeration, but is more lightweight than garden soil.

Just to throw a wrench into things, potting mixes—also known as soil-less mixes—don’t contain soil at all. Instead, these are made up of non-soil additives, like peat moss, pine bark, and mined perlite and vermiculite.

Recently, gardeners and potting soil manufacturers alike have been experimenting with ‘greener’ non-soil additives, including PittMoss, a growing medium blend made from recycled paper fibres.

Some gardeners are quite fussy when it comes to garden soil vs potting soil—particularly when growing plants in containers. In very large, outdoor pots, garden soil may be more economical. As you become familiar with some of the most common ingredients found in these products, along with their pros and cons, you should be able to make better purchasing decisions. You might even have a go at mixing your own custom garden and potting soil blends, too.

Marilyn writes regularly for The Portugal News, and has lived in the Algarve for some years. A dog-lover, she has lived in Ireland, UK, Bermuda and the Isle of Man. 

Marilyn Sheridan