If I had planned ahead I would have done solar pasturization by putting saran wrap on the top of a picnic cooler full of compost.  Since I didn’t plan ahead I did this instead:

Grilled dirt anyone? The oven bag makes it look kinda like a dirt haggis.

An oven bag full of  my best screened compost cooked well done on the gas grill.  I added about a quart of water so that it would all steam evenly, and punched a small hole in the top to keep it from building pressure – took about 2 1/2 hours to reach 170 degrees Fahrenheit.  The probe thermometer lets you be more efficient by not opening the lid.  Needless to say plan on just letting it sit there for several hours to cool.

If you only cook the compost you should be able to make 2-3 times this volume of sterile homemade organic potting soil by the time you add the other ingredients.

The oven bag looks no worse for wear and tear, and I don’t see why you couldn’t reuse it again – for dirt of course.  Next time though maybe I’ll do better and use sunshine instead of fossil fuel, but in the middle of January this worked pretty well without cooking dirt in the kitchen.

This is the rig that I use to screen compost – it works very well and is built out of scrap lumber and a piece of 1/2″ by 1″ galvanized wire mesh left over from building some rabbit cages.

The last time I painted the garage roof I gave my retired commercial wheel barrow a nice thick coating with the last bit of roof paint to keep from having leftovers.  It’s ugly, but it keeps it from rusting.

A simple efficient compost screener

Just work your compost around with a hoe and pull the big pieces off the back.  Toss them back  on to the compost pile when you finish.

Work it about and pull the big pieces off the back

In a few minutes you’ll have a nice bit of screened compost.

This already looks like potting soil

If you want to make sterile potting soil you have to pasteurize  the compost by heating it up enough to kill any organisms that it might contain.  I don’t do that, and I rarely have any soil related problems because I make my compost in such a way that it heats up to over 150 degrees in the pile.  This apparently kills pretty much all of the pest eggs,  weed seeds and disease pathogens because I’ve been using it like this for years. If I wanted to heat pasteurize my compost or soil I would do it like this – Solar Heat Pasteurization – or in an oven bag on the grill – not by baking it in the kitchen oven as I have seen recommended.  My wife wouldn’t be pleased if she caught me baking dirt in the oven.

I mix and store potting soil in a clean 30 gallon trash can. I also use steel trash cans to store bird seed, and pet food (I usually leave those in the sack inside the can) They’re not too expensive and they’re fairly effective at keeping out mice and other vermin.  However we had to fasten the lid down with bungee cords last year to keep the raccoons out of the cat food.

Layer in your ingredients.

Layer in equal measures of screened compost, peat moss (or coir), and course vermiculite or perlite, and a bit of organic fertilizer if desired.  You might also want to add some lime to lower the acidity, but I usually don’t.  Shop around for your components – I save a lot of money by buying big bags of vermiculite from a commercial greenhouse instead of little bags from the home improvement center.  If you buy compost instead of making it beware of “compost” that is really just rotted bark mulch – it has almost zero nutrition, dries out quickly, and ties up a lot of nitrogen as it continues to break down.  Bark mulch is bad.

Every few layers (1/4 – 1/3 ways through filling the container) give it a good stir.  If you wait until it’s all full, it’s really quite difficult to mix it all up.

Stir it up several times as you go.

Once you’re finished mixing it all up add about 3 gallons of water.

I add about 2-3 gallons of water, and in a day or two it will all be slightly moist, but not wet.

Adding some water to the container makes the mix less dusty to handle and prepares it to take up water.  Now you’re ready to start some plants.

I’ve used a similar mix for making square foot gardens – basically raised beds.  It’s wonderful to work with, but in hot weather it dries out awfully fast, and I think that a mix which includes some clay or garden soil might actually be better for that application.

You can use many other ingredients for making perfectly usable potting soil – including plain old dirt.  Don’t let a lack of specific soil ingredients hold you back – seeds were coming up just fine long before anyone ever dreamed of buying or selling dirt.

Make up a batch of potting soil now, and you’ll find it easier to get around to the seed starting and potting that you plan to do later.

More information on making organic potting soil – includes many recipes and information on alternative ingredients.