How to Sterilize and Reinoculate Soil

Looking for instructions on how to sterilize and reinoculate soil? You found 'em! Let's talk about the whole process of renewing your soil for re-use.

Why Would I Sterilize My Soil?

You should sterilize any soil that you've used previously for seed-starting. This is because seed-starting conditions are extremely wet and allow fungus (such as the one that causes damping off) to thrive.

You should sterilize any soil that contained diseased plants.

Consider sterilizing any soil that's contaminated, diseased, or infested with pests or pest eggs.

Additionally, sterilize any soil that's just super dang questionable, and you aren't sure if you trust it.

How to Sterilize Soil

Stick it in the oven. NOT TOO HOT, though! Soil can release some downright NASTY fumes if you bake it at a high temp. Noxious, poisonous stuff that shouldn't be vaporized in your home. Seriously, heed this warning.

Do Not Bake Soil Above 200f (93.3C) Indoors!

Cool, now that's out of the way - put your soil in a pot or crock large enough to hold it, and small enough to fit in your oven. Crank that thing up to 180F (82.2C) and bake the soil for at least 6 or 8 hours. Your house may smell a bit earthy, but nothing crazy.

An alternate here is to lay your soil out flat and thin on one or more baking sheets. This allows you to reduce the cook time down to just an hour or two. Increasing the surface area drops the cook time.

Let the soil cool thoroughly in the oven after it's done baking, and you can consider it to be pretty sterile at this point. You've likely killed most or all of the fungus, bacteria, eggs, critters, mites, etc. in there.

Next, you'll want to reinoculate the soil to bring it back to life.

How to Sterilize a Lot of Soil With Solarization

If you got more soil to sterilize than you can fit in an oven, I've got just the solution for you.

Soil Solarization

Soil solarization involves covering your soil with a black or clear plastic and allowing the Sun to heat sterilize the soil underneath for 6 to 8 weeks.

Soil solarization times are highly climate dependent however, and you can solarize soil in a much quicker time in the desert Southwest then you can in a temperate forest.

Soil solarization times are highly seasoned dependent. You can solarize soil faster in the summer then you can in the winter (in the northern hemisphere).

The most common method for soil solarization is tarping. This involves laying a tarp or piece of landscaping fabric or clear plastic over the soil to be sterilized and leaving it for quite a few weeks.

I've also seen farms use some sort of giant black plastic bio bag for soil solarization, though I'm not sure exactly what the product is.

What Does Inoculating Soil Mean?

Soil that has been baked is sterile. Sterile soil is not at all helpful for growing organically.

You'll want to inoculate your soil with microbes, beneficial bacteria, mycorrhizae, and good bugs. Inoculating your soil means that it will come back to life and start growing again.

Why Should I Reinoculate Soil?

You should reinoculate your soil to bring it back to life if you sterilize it.

Reinoculating soil is an important step to prepare your soil for use again. Don't skip this step!

How Do I Reinoculate My Soil?

The best way to inoculate sterilized soil is to add it to your compost bin. There, all the good bugs you've been feeding and growing will be able to colonize the sterilized soil and it will come back to life.

Remember here, the more sterilized soil you add to your compost, the longer it will take to become fully colonized. Try to add smaller amounts of sterilized soil, so inoculation happens fast.

If you don't have a compost bin, you can add beneficial microbes to your soil as an inoculation, and water them in. This will take longer than a compost bin, and you'll end up with less diversity in your soil biome, but it will do the trick.