Most potting mixes are a blend of two kinds of ingredient: some that hold water and some that keep the mix open to air. Knowing which is which lets you read a bag or adjust a mix.
Water holders. Peat moss is light, holds a lot of water and is naturally acidic, but it is harvested from bogs that store large amounts of carbon, so many gardeners now choose peat-free. Coconut coir, made from coconut husk, holds moisture well, rewets more easily than peat and resists compacting; buy coir that has been washed, because unwashed coir can carry salts. Compost adds some nutrients.
Air makers. Perlite, the white popcorn-like granules, and bark chips open up the mix so water drains and air gets in. Grit or sharp sand adds drainage for plants such as cacti; the RHS suggests heavier loam-based mixes to steady top-heavy plants. Perlite and sand add little or no nutrition.
So if a plant dries out too fast, a mix with more coir or peat helps; if it stays soggy, add more perlite, bark or grit.
Key points
- Peat, coir and compost hold water; perlite, bark and grit add air.
- Choose washed coir to avoid salt problems.
- Adjust the balance to how wet or dry a plant likes to be.
Try this
Take a handful of your potting mix and spread it on paper. Pick out the pieces you can recognise: fibres, bark, white perlite, grit.
Check yourself
Read more in the reference
Sources
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