What is potting soil?
Technically, commercial potting soil is not soil at all, but a blend of materials like composted bark, perlite, sphagnum moss, vermiculite, palm coir, river sand or fine grit and compost. The ideal potting medium is light and loose so that plants can easily root into it, and airy enough to allow enough oxygen to reach plant roots. It is odourless and has good drainage, but is also moisture retentive. It should contain some nutrient value, must be free of disease and weeds, and should not break down too fast or become compacted and difficult to rehydrate.
Can you make it at home? Although it is convenient to just buy all-purpose commercial potting soil, it can become an expensive exercise if you have to fill up a lot of containers. Making up your own ‘signature mix’ in the old wheelbarrow is also fun to do, and you can experiment with and tweak your recipe as you go along. Basic recipes Potting soil recipes are a dime a dozen, varying from the heavy old-fashioned loam-based mixes to modern-day mixes enriched with organic fertilisers, water-retention products and soil conditioners.
- The palm peat comes in a brick that should be hydrated in about 3 – 4 litres of water in a bucket beforehand, then broken up and fluffed out, and squeezed dry of excess water. Sphagnum moss was previously used but is not sustainable, hence the more eco-friendly palm peat, which works equally well.
- Perlite resembles polystyrene balls but is actually made up of fired volcanic rock. It aids drainage and reduces the weight of the soil.
- Vermiculite consists of silvery-grey particles. It is a natural volcanic mineral that has been expanded with heat to increase its water-holding capacity. It will not deteriorate or lose volume and adds calcium and magnesium to the soil.
Fine compost adds nutrients and beneficial microbes. You can either sieve well-rotted home-made compost or buy bags of compost, which are cheaper than bags of commercial potting soil.
- Composted fine pine bark releases nutrients as it decays, and also aids drainage. It can be used as an alternative to compost.
- Coarse river sand helps with moisture retention and aids drainage.
- You can use Atlantic Bio Ocean fertiliser, which contains seaweed, fishmeal, humic acid and composted poultry manure.
- If you do not yet have a worm farm to harvest worm compost from, use Efekto Wonder Vita-boost, which contains vermicompost.
- 1 bucket palm peat
- ½ bucket perlite ½ bucket fine compost or fine composted bark
- ½ bucket vermiculite
- 2 cups of river sand
- 2 cups of slow-release fertiliser
Mix this all together. If it feels too dry, lightly moisten it with a watering can. Left-over medium can be stored in a bin with a lid in a cool dry place.
- 1 part palm peat
- 1 part vermiculite or coarse river sand
- 2 parts fine compost
- ½ – 1 cup worm castings or worm compost
Why not use garden soil?
Although sieved and sterilised topsoil can be part of a heavier potting medium, it is not advisable to just dig some garden soil up to use in a pot. It could be full of weed seeds, carry pathogens and disease, may drain badly (causing roots to become waterlogged) and it will become compacted.