Potting soil is sterilized to give your plants the best growing 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)

• Fill an ovenproof container about 3 inches deep with soil, mix in a generous amount of water (not enough to make it runny or soupy but thoroughly wet) then cover with aluminum foil.
• Bake in a preheated oven (200°F) until the temperature of the center reaches 180°F (use a meat thermometer to measure). Once the temperature reaches 180°F, bake for 30 minutes.
• Do not overheat or overbake since it can release toxins harmful to plants as well as kill beneficial organisms.
• It can smell quite foul when baking, this is normal.

Microwave Method: (small batch)

• To use the microwave, put about 2 pounds of moist soil in a thick, plastic bag. Leave the top open and place it in the center of the microwave.
• Heat it for two to five minutes on full power, checking the temperature in the middle of the dirt with a thermometer. When the target is reached (180°F to 200°F), close the bag carefully and put in a cooler to hold the heat in.
• Allow to cool. Source: tcpalm.com ^[1].

Sun Method: (large batches)

• Choose a spot in the yard that receives at least 6 hours of sun during the day (8 hours a day is best).
• Lay out clear plastic sheeting and cover with a layer of dirt about 4 inches deep. Spray generously with water (not so much that it becomes runny muck).
• Cover with another sheet of clear plastic and secure the plastic in place by laying a border of rocks all along the edges of the plastic.
• Bake in the sun for at least 4 weeks in hot, sunny weather and up to 6 or 8 weeks in cooler weather (this technique is only good for summer).
• Tip: Rake up the dirt each week to make sure the heat reaches all of it.

Tips:

• Reusing potting soil is fine when using with mature plants, though new seedlings or bedding plants require sterilized to have the best chance to thrive.
• Don’t use dirt straight from the garden for a potting medium (alone), mix it with other ingredients to make it lighter and more beneficial for your plants (see below for a few recipes).
• After completing one of the methods above, the dirt will likely be hard and clumpy. Break it down first before mixing with other ingredients.
• Make sure that potting containers themselves are clean since they can also harbor disease organisms. You can wash the containers in a bleach and water solution or see this page ^[2] for more suggestions.
• You can use these same methods for used potting soil and sand.

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:

• Add 1 TBS blood meal, 1 TBS kelp, 1 TBS greensand, 2 TBS bone meal to 1 gallon of potting mix. Mix well. Source: sunset.com ^[6]