Bagged potting compost is so convenient isn't it? Cut open a bag, chuck a few handfuls into a pot, bung in your plants, and that's it. Most plants will grow well in it without a problem, but if you've ever bought a bag of stale potting compost (loam-based potting composts are especially prone to going 'off'), or if you've been less than satisfied with a crop's performance using commercial brands, it's worth having a go at making your own.
I have three basic mixes that I make up fresh whenever I need them, and which can easily be tweaked to make them the perfect growing medium for specific crops. All you need is a potting scoop or a container for measuring, a container or tray for mixing (plus an extra container for soaking if you're using coir), and a few basic ingredients.
Easy Home-made Seed Starting Mix
When starting seeds, it's a waste to use your best garden-made compost. Seeds don't need a rich growing medium in which to germinate – the seed itself contains its own supply of nutrients that enable the plant to burst into life.
My seed sowing mix is made up of just three ingredients in equal parts – it really couldn't be simpler and works well for me. The first ingredient is loam, which you can buy in bags or, alternatively, make with turf lifted from the garden (for instance when making new beds). I stack my turves upside down under an old builder's bulk bag for a couple of years until they're well broken down. The resulting loam is then sieved to remove any surviving weeds or other detritus before use.
Commercial loam-based potting composts also use stacked turves for the loam component, which are sterilised to kill off any weed seeds, pests or diseases. It is possible to sterilise soil in the oven at home or even in the microwave, but unless you only have a very tiny quantity of soil to sterilise, this just isn't practical for most of us.
The second ingredient is coarse sand, which improves drainage and aeration in the mix. This is important because soil used on its own has a tendency to collapse when it becomes wet, reducing the pore spaces in the soil and squeezing out air, resulting in poor plant growth.
My preferred third ingredient is leafmould, which has a wonderful texture and retains moisture well. Most years I don't produce enough leafmould from my small garden for all my needs, but coir or composted bark make acceptable substitutes. Coir is usually supplied in compressed bricks that need to be soaked before use – I use a large, flexible, rubber bucket for this. It doesn't take long for the water to soak through the brick and, as if by magic, turn it into a lovely, fibrous, reddish-brown material that's ideal for potting mixes.
It's best to mix the ingredients together with your hands so you can easily feel lumps and break them up as necessary, but for larger quantities you can use a spade.
Best All-Round Potting Mix
For growing plants on, I like to use a different three-part recipe. Growing plants need more nutrients than seeds, but not too much – sturdy, steady growth is the aim. This mix is again based on loam from stacked turves, which is reasonably heavy and therefore helps to stop pots from blowing over, especially when growing tall plants. Loam is not rich enough to grow plants in by itself – they'll quickly exhaust the supply of nutrients in the constricted space of a container – so it needs to be mixed with other ingredients.
Leafmould makes up the second part, or composted bark if leafmould is scarce. While neither of these adds much in the way of nutrients they do have good water-retaining properties and will help to maintain good structure in the potting mix. They are also full of microorganisms that do a great job of suppressing diseases. Coir is also fine to use.
The third part of this recipe is compost. Ideally, your own garden compost, which will be rich and fertile, though bagged commercial compost can be used for this part. When using your own compost you won't usually need to add extra fertiliser for most crops.
Rich Potting Mix For Hungry Crops
Greedy crops such as tomatoes and peppers need a potting soil that can provide plenty of nutrients over a long period, so my rich potting mix comprises just one part leafmould, coir or loam to three parts garden or bought-in compost. This will be sufficient to sustain these plants until fruiting time, when additional organic liquid fertilisers are needed.
There is no one-size-fits-all potting mix, so the recipes above are just starting points. It's important to experiment and try different quantities and materials to find out what works best for you and your plants. The beauty of making your own potting mixes is that they are infinitely customisable. For example, if you want a mix that drains more freely, add some coarse sand or perlite. If you want a richer growing medium, incorporate well-rotted manure or wormcasts, or use comfrey leafmould in place of plain leafmould.
What are your best tips for the perfect mix? Share them with us by leaving a comment below.
By Ann Marie Hendry.