Homemade potting mix: not as hard as it sounds

22:44, Nov 13 2012

This year, I am planting my tomato plants in pots and not in the main garden. The reason for this is because I'm being extra cautious with this year's babies. It's make or break year - if I don't get a decent crop, I'm swearing off tomato-growing for life. 

Tomatoes need heat and sun and shelter, and I'm not quite sure where the best place for that is yet. Since the great tree chopping exercise, there is so much more sun on the north side of the house, it'll be the perfect sunny spot for tomatoes to bathe, but I'm not sure what the wind will be like through there. If I put the toms out there and it's not as sheltered as I'd like, I can move them somewhere a bit more hospitable. Also, being in pots should mean the soil stays a bit warmer.

What medium should an organic gardener on a limited budget use to grow their potted tomatoes? Regular soil isn't quite right for pots - it doesn't drain well and water builds up. Air can't get in and plants suffocate. Bagged potting mix from the garden centre is an easy option, but they inevitably have a slow-release chemical fertiliser in them, and I'm not down with that. It's also kind of expensive. 

I looked for alternatives on the market. I called Living Earth to be told they don't yet sell their organic potting mix in Wellington, but found Dalton's at my local Bunnings. It cost a whopping $16 for a 40l bag, and has a fertiliser in it called NuFert. The website doesn't tell me what NuFert is exactly, but it's chemical free. Still, at the price using bags of this stuff would raise the cost of my tomatoes so much, I may as well buy them from a shop and save myself the agony. 

It looked as if I was going to have to come up with a homemade solution. There are lots of recipes on the internet, mostly from American sites using ingredients like perlite that aren't readily available in New Zealand. Most recipes seemed to have an ingredient that helps retain water, an ingredient that aids drainage and lets air pockets in the soil so roots can breathe, and nutrition. The most basic recipe I could find consisted of compost, loam, perlite and peat. 

Peat is commonly used to help retain water, but as this article will tell you, peat may be a renewable resource but, like oil, it takes more years to renew than you can shake a stick at. I chose to use leaf mould in my recipe. 

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I don't have loam just lying around to use - my soil is predominantly heavy clay and anything that isn't is being used to grow food right now - so I used only compost for the nutrition part of my mix. 

And then the ingredient that aids drainage - this is what perlite does in all those US recipes. I used a bag of pumice that I collected over a year ago and crushed it up. It was hard work, and I broke a plastic tub doing it, but I felt pretty great thinking I was using something for free, and getting bigger guns too. You can get pumice from the garden centre if you don't have it handy. There's also horticultural sand or grit that will do the same job.

So here it is: my recipe for potting soil. 

2 parts compost - if it's homemade, you might need to sieve it to get a finer texture

1 part crushed pumice

1 part leaf mould, the finer the better

Some blood and bone, chucked in at the rate recommended on the packet.

I mixed it all up in a wheelbarrow and threw in a few banana peels left over from lunch. They should give the tomatoes a hit of potassium, which helps with fruiting.

But wait, there's more. My homemade tomato fertiliser will also go in each pot. It's just a big jar that I've been filling with coffee grounds, crushed egg shells, hair (yes, it's actually my own), and banana peels. In the interests of science, I will add this fertiliser to all except two of my plants, in an attempt to see what difference it makes.

Do you have strategies for growing great tomatoes? 

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