DIY Potting Soil

Potting soil is big business. Every year thousands of bags of potting mixes are sold from nurseries and garden centers across the country.


Potting mix is particularly appealing to gardeners for a couple of reasons.

First, it’s sterilized. Whether you’re buying peat moss potting mix or a composted bark potting mix, the concoction has most likely been sterilized through steaming or some other heating process, or (at least historically) through a chemical process. One of the biggest frustrations for seed-starters is seeing young seedlings topple over and shrivel up. This is caused by damping-off fungi, which are present in large amounts in soil. Using sterilized potting mix prevents this problem.

Second, usually nutrients have been added to help your plants get a good start. Though most of the original nutrients are destroyed during the sterilization process, a lot of potting mixes now contain added nutrients, which should provide seedlings with all they need to make a healthy start in growing.

It all boils down to convenience. But what in the world did gardeners do before bagged potting mixes?

They made their own. And you can too with a little extra work and planning, and it can save you a bundle of money, especially if you’re growing en masse.

soil and good compostMake Your Own

We have used well-sifted compost before, and loved the results. It must be fully composted and sifted so that nutrients are available and the seeds won’t get lost amongst big bits of compost. It can be composted leaves, garden refuse, wood chips… pretty much anything organic that has been broken down into a fine, clean compost.

You can re-use potting mix you’ve already bought.

Vermicompost is also an excellent seed-starting medium.

Or you could simply use a good, loamy soil that doesn’t pack (aka, not clay). In old days, they used to start seeds on all sorts of mediums. One of the favorites was the dirt from mole-hill mounds because of its lightened texture. (If you’ve got a mole problem, at least you can put that mole-hill dirt to some good use.)

Sterilizing Your Own Potting Mix

To be sure that your seed-starting efforts won’t be thwarted by damping off, which is likely to be present in any compost or soil you might use, you’ll need to sterilize it yourself. We’ve done it a handful of ways.

1. Use your oven. This is the easiest and fastest way of sterilizing the soil, and though you can only do it in small batches, you already have the space and equipment to do it. We use the full-size catering pans (you can get them at many grocery stores) and simply fill it with sifted compost or soil, add a bit of water to mix it around to make sure it’s slightly damp (this steams it, which helps with the sterilization process), and then stick it an oven preheated to 170〫to 200〫F. The soil needs to get up to at least 160〫F.

Photo credit: UNDP

Photo credit: UNDP

2. Solarize it. If you want to do it in bigger batches, using solar power is one way to do it. You can use a solar oven to heat up the potting soil. (Make your own.)

Or, if you have a metal or dark-colored compost tumblr (preferrably one that hasn’t been used to actually compost the compost) you could put the sifted, slightly damp homemade potting medium in the composter, and let the sun bake it hot for several days. You’ll have to wait a little longer, but you’ll get a lot more done at once.

OR, an even more cost-effective, solar-powerized option is to simply place the sifted compost in clean black garbage bags (double-bagged to retain the heat) and let it sit in the sun for a few days. To make sure it gets heated all the way to the center, it would be better to only fill the garbage bag halfway, and lay it out flat.

3. Fire power. If you have a fire-pit or backyard pizza oven, you could do large batches much faster, as long as you have a container that will hold the potting soil.

Remember to keep an eye on your seedlings and make sure they have the nutrients they need to grow. Once they’re large enough, mulch them well.

Are you thinking about getting a compost tumbler? You could buy one, but honestly, I would recommend trying other methods first. Compost tumblers are expensive and, in my opinion, not cost effective.

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