I’ve been frustrated with commercial potting and seed raising mixes in recent years. Not only are they expensive, but my vegetable seedlings languish in them, if they germinate at all.

The culprit is most likely clopyralid herbicide residues in the mixes. Even ‘organic’ compost may have traces of the herbicide in it, because the chemical doesn’t break down easily, and can be found even in manure from animals that have eaten plants sprayed by it.

So this year I decided to try making my own potting mix. The results have been encouraging.

My biggest hurdle to making good potting mix is ridding my compost of weeds. I don’t do hot composting. Though some of my pile will get to the temperature necessary to kill weed seeds, not all of it does. Fortunately, I have an effective way of sterilising compost—the bread oven.

This spring, every time we’ve fired up the bread oven, I’ve used the residual heat, after all the baking is done, to sterilise compost. 

I put moist, sieved compost into a restaurant steam tray (lidded) and/or a stock pot (lidded) and put it in the hot oven until the temperature in the centre of the compost reaches 82℃. This takes a couple of hours, as the oven is usually only around 150℃ by the end of baking. 

I mix my sterilised compost with coarse landscaping sand in a 2:1 ratio, and voila—my own potting and seed raising mix!

Sterilised soil is prone to fungal outbreaks, because there are no other microorganisms to keep the fungi in check, so when I use my mix, the first watering I give it is a slurry of soil from the garden. This inoculates the mix with the healthy mix of microbes from the garden and avoids excessive fungal growth.

And how did my mix do, compared to commercial mix? Spectacularly well! 

Because I didn’t decide to make my own mix until I actually needed it, some of my seeds were planted in commercial ‘organic’ mixes. Many of these seeds failed to germinate this year. Those that did germinate then sat without growing at all until I transferred them to my own mix.

Seeds planted in my own mix germinated well and grew vigorously.

I will definitely be making my own potting mix from now on.