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How To Sterilize Soil & Organic Potting Mix Recipes

Potting soil is sterilized to give your plants the best environment possible by killing weed seeds or disease organisms that might be lingering in the soil. Most commercial products have already done this but if you want to use that rich dark gold right from your garden or reuse what you have, here are a few different ways you can do it yourself.

You’ll also find a few different recipes for making your own organic potting soil (found at the bottom of this page).

Oven Method: (small batches)

Microwave Method: (small batch)

Sun Method: (large batches)


If you want to make your own organic potting mix, you still have to avoid using any prohibited ingredients and that means checking out all the individual ingredients for their organic acceptability.

It may surprise you to learn that products like peat moss or limestone are sometimes treated with prohibited materials such as wetting or anti-caking agents, so don’t rely on assumptions about purity.

In addition to meeting certification requirements, your final product will also need to provide plant roots with the right amount of air, water and nutrients.

(Source: Potting Mixes For Organic Growers [3]).

Classic Soil-Based Recipe

1/3 mature compost or leaf mold, screened
1/3 garden topsoil
1/3 sharp sand

For Styrofoam Seedling Flats

2 parts compost
2 parts peat moss
1 part vermiculite, pre-wet

For Sowing

5 parts compost
4 parts soil
1-2 parts sand
1-2 parts leaf mold, if available
1 part peat moss, pre-wet and sifted

Note: All ingredients are sifted through a 1/4-inch screen. For every shovelful of peat, add two tablespoons of lime to offset the acidity.

For Pots and Baskets

30 percent topsoil
60 percent peat
10 percent perlite
5 pounds lime per cubic yard
3 pounds dolomitic lime per cubic yard

Note: The handling of this pot mix is the same as for pack mix.

Bedding Plant Recipe

25% compost
50% peat moss
25% perlite or vermiculite

*Source: National Sustainable Agriculture Information Services [4] (first published February 28, 2007 and moved to this page for better organization)

Looking for a diy seed-starting mix? Try this recipe (Source: organicgardening.com [5]):

4 parts screened compost
1 part perlite
1 part vermiculite
2 pars sphagnum peat moss (or coir)

Controlled Release Recipe: