How to make your own potting mix

Spring is all around us down under in southern Australia and the nurseries have been crowded for the past six weeks as folk stock up on vegetable and flower seedlings. But at $5 a punnet (six small seedlings) I can no longer afford to garden in this fashion on the scale that I do; the price of our vegetables from the garden would become unconscionable.

So I’ve bided my time, saved my seeds and waited for Mother Nature to signal that the soil temperature is warm enough to begin to plant out seeds for my summer crop; the first tiny green leaves on the chestnut tree have flushed out and let me know that it’s time to make the potting mix.

What exactly is potting mix? Well, the stuff in plastic bags that can be purchased from your local nursery has only a very small amount of soil in it and a whole lot of free-draining materials like sand, coconut fibre, peat, or ground bark because there’s a big emphasis on not drowning the seedlings if you over-water them, as happens to anxious parents of young vegetables in pots. Then there’s the water-holding materials like perlite or vermiculite, wetting agents to get the stuff to absorb water in the first place, and some slow-release fertiliser thrown in to feed the seedlings. Much of the initial weight is water. Still, nothing like what Mother Nature provides on the forest floor for seeds that fall out of the canopy above to start the next generation of plants.

But there’s no need to purchase potting mix to raise seeds; its really not that hard, as seeds are as anxious to get going as you are to have them do so. There are plenty of sources of good friable soil suitable for the purpose around the garden if one knows where to look. The bottom of the compost heap lying on the soil is an especially rich source, but there is one other source where ready-made potting mix can be found; the garden paths. Provided that you’ve covered your paths with wood chips rather than pavers or concrete, walked over them for a few years and let rain and sun work their slow and gentle transformation, potting mix is available underfoot whenever you need it. The broken-down bark mixed with soil will provide the right combination of drainage and water retention plus humus to feed the seedlings. You just need to trust nature to get the soil balance right and to trust that your seedlings will fight off any soil-borne fungi, weeds or other diseases.

Whatever your source of soil, a wheelbarrow and large sieve (mesh size about 5 mm), a shovel and a strong back are all that is needed. Shovel the degraded chips and soil mix into the sieve on top of the barrow, rock it back and forth sideways over the barrow, and there’s your potting mix. Throw the large bits left in the sieve back onto the path for the coming years, or replenish it if needed; mulch on paths is not only soft underfoot, but keeps the soil between the garden beds cool and moist, forming a reservoir for worms and tree roots and a natural catchment for rainfall. The soil in your paths is ready for use as potting mix when self-seeded lettuces can be found growing there after winter rains.

Seedlings should be grown above ground – this keeps the chooks out, but also earwigs and caterpillars and other things that creep out of the soil and garden beds at night time to feed on tender seedlings. It also saves you bending over all the time when watering or weeding them; pure rainwater from a watering-can works best for me, as it contains no salt.

Wheel the barrow over to where your seed table is, get the seed tins out and a small trowel, some plant labels and a black permanent marker pen. This pen is about the only thing I need to bring in, and I admit that I just get one of them out of the stationary cupboard at work. The wooden markers I cut out of old flat-slat wooden blinds, and cut a point on them with a scroll-saw. (OK, I might be too stingy to buy potting mix, but I spend all the money I save on good tools!) 

The seedling containers are actually seed trays; I can’t be bothered with those tiny punnet containers that would require me to drop a single seed in each tiny compartment. I lay my home-grown potting mix loosely about 25 mm deep into these trays, sprinkle seeds roughly on top, and rub handfuls of more potting soil over them, patting them down loosely to seat the seeds against the mix.

Does it matter about seedling spacing and all that? Not to me. I’m going to pull them apart anyway when I transplant them and as I’ve plenty of seeds I prefer a little wastage of seedlings to fiddling about at this stage (with seeds that may not necessarily be viable) by planting them out en-masse.

Two hours later I’ve gone from ground-zero to a table full of some hundreds of seedlings. After all, this is the easy bit – the real work comes in six to eight weeks time when I have to plant them out on hands and knees… Now they will stand in this sunny spot and I’ll water them daily, keeping the blackbirds off with some wire-mesh cloches. And as there will usually be plenty of seedlings left over, I can endear myself to fellow gardeners by gifting them handfuls of seedlings wrapped in wet newspaper that would otherwise cost them tens of dollars down at the nursery.