Have you ever wondered how they did it in the old days, before the times of store-bought pre-bagged potting mix? Knowing how to make your own potting soil from scratch not only saves you money, but is much more convenient and often healthier for your plants than commercial stuff.
The main ingredients in homemade potting soil are: compost, garden soil, leaf mold and/or rotted saw dust, and wood ashes. Simple, right? I’ll tell you the recommended proportions in just a minute.
First, if you don’t already have a compost pile started you’ll need to begin with that.
Create A Compost Pile
Building a compost pile can be an extremely scientific calculation of carbon to nitrogen ratios, or it can be as simple as knowing your browns and greens. I tend to prefer not to complicate things.
To build a healthy compost pile, you’ll need a variety of natural materials which you will layer 2 parts green to 1 part brown.
Examples of “brown” materials are:
- sawdust (from untreated wood)
- shredded newspaper (not the glossy colored pages)
- nut shells
- pine needles
- corn stalks
Examples of “green” materials include:
- aged chicken manure
- coffee grounds
- fruit and vegetable scraps (fresh, not cooked)
- fresh grass clippings (not from a sprayed yard)
- freshly pulled weeds
- well aged/rotted livestock manure
- Hummus (soil from the forest floor)
- garden waste (no pesticides, etc.)
You’ll wanna check out this fantastic infographic on composting to show you in detail how to make compost with these simple ingredients.
As the materials become available to you, try to add them to the pile in the 2:1 ratio. If you notice your pile getting pretty thick with browns, make sure to balance it with some greens. An unbalanced compost pile will either sit there and do nothing, or it’ll rot and begin to stink like crazy.
As you build your compost pile and allow it to sit overtime, microorganisms will go to work breaking down all of those natural materials and turning them into beautiful “black gold”. Be patient! It can take a couple of years to get a good pile of compost established. (For more information, I think you would enjoy reading David Goodman’s “Easy Composting” article. Great tips!)
Once you have a sufficient amount of fresh compost to work with, you’re ready to start mixing your homemade potting soil.
Now that you’ve created a lovely, rich compost, the next step is to screen it. You can build a simple wooden frame and use 1/4″ screen to sift out large chunks of debris, twigs, and leaves from your compost to get a finer end product to work with.
There’s a great DIY Compost Screen/Soil Sifter that you can build at home to make this process much easier!
Pasteurizing Your Compost
You can use compost straight from the pile, but doing so risks introducing fungi and ungerminated weed seeds to young seedlings and house plants. Older plants which are ready to be transplanted into the garden or are already in the garden will benefit from the microorganisms in unheated compost, however when you are starting seeds you don’t want anything to interfere with the germination and sprouting process.
It’s best to pasteurize the compost before combining it into a potting mix by heating it to 180* for 30 minutes. It’s important not to allow the compost to be heated over 190*, or else it may develop compounds which hinder the growth of your plants.
To pasteurize your compost in a solar cooker, fill the largest pot that will fit in your solar unit with compost, place the filled pot in a sealed garbage bag, and allow it to heat for several hours- keeping an eye on the internal temperature of the box, and not allowing it to get over 190*.
To pasteurize your compost in an oven (electric, propane, or wood cookstove), fill a large casserole dish or a dutch oven with 3″ of compost. Lightly moisten it with a little bit of water mixed throughout, leaving the compost loosely fluffed. Cover the dish with aluminum foil so that no steam can escape; poke a meat thermometer into the center of the foil lid at an angle so that you can read the temperature without having to pull the soil out of the oven.
Place the pan in the oven to cook. Keep a close eye on the thermometer. When it reaches 150*, turn the oven off. Continue watching the temperature. When it gets to 180*, carefully remove the pan or pot from the oven and wrap it in several layers of thick blankets or towels. Transfer the wrapped dish to a cardboard box for best results, covering entirely with a cloth to continue a slow cooking process.
When the dish is completely cool, transfer the pasteurized compost to a clean bucket to be used as needed.
Another easy way to pasteurize large amounts of compost at once is to place the pile in direct sunlight and cover with a thick black sheet of plastic. Allow to sit for a couple of weeks. The sun will bake the soil, germinating unsprouted weed seeds which are then smothered under the plastic cover. You can hand pick or turn over stubborn weeds beneath the plastic.
Collecting Good Garden Soil
Once you have a nice, clean compost ready to mix, you’ll need to collect garden soil. You want to choose from the best soil in your garden to make a rich potting mix. Make sure to screen out any rocks, roots, or other large materials.
You’ll want to sanitize garden soil just as you did the compost. Spread the soil 3″ deep in a tray and bake at 200* for 20 minutes, stirring every 5 minutes.
Mix 5 parts compost with 4 part garden soil. If you will be using the mix to transplant healthy seedlings, you might make it more of a 50:50 ratio. You may have to experiment a little to find which blend works best for you.
Add Moisture Retention
Commercial potting mixes typically use Spaghnum peat moss for moisture retention. You can easily substitute aged leaf mold or rotten (untreated) sawdust from home, or a mixture of the two.
If you are just beginning a compost pile, now is also a good time to work on a making leaf mold. Make a pile of leaves 5 feet high, and allow it to break down naturally over time. Build a three-sided bin to keep the leaves contained. It can take up to 2 years for leaves to fully turn to leaf mold. You might toss the pile every so often to keep it mixed up.
Add enough leaf mold or rotted sawdust to the potting mix to allow for good water retention throughout; 1-2 parts leaf mold or sawdust, or a combination of the two.
Don’t Forget Drainage
To ensure good drainage, you’ll also want to toss in 1-2 parts sand. Course sand (almost pebbly) gathered from a local source instead of store-bought sand would be the best option. If the sand is too fine, it will fill the air pockets in the soil and actually reduce drainage.
A Dose of Vital Minerals
To top off your homemade potting soil, sprinkle some wood ash over all and mix in well. This will help add calcium, potassium, magnesium, and trace minerals. It is a great alternative to commercially sold lime. Wood ash raises the pH in the soil, helping to neutralize acidity levels. Hardwoods produce ash with more potassium than softwoods.
Homemade Potting Soil Recipe
5 parts compost
4 parts soil
1-2 parts leaf mold
1-2 parts sand
2 Tablespoons wood ash per shovelful of potting soil (or as needed depending on your soil’s acidity levels, which you can have tested at your local county extension office.)