How to Pasteurize Potting Soil

A couple of weeks ago, I came across an article by Ashley over at Practical Self Reliance on growing lemon trees from seed.  I think I had tried growing a lemon tree indoors about 20 or 25 years ago; I wasn’t very successful.  I’m not really into houseplants.  I tend to forget to water them.  Especially when little children and all their attendant “emergencies” surround me.  They always took priority.  But now there’s only one “little” child left at home, and she’ll be leaving in less than six months, assuming the world holds together.  (And I’m not all that sure that will happen.)  I got some grow lights last month and dug up several pepper plants to bring inside for winter, with hopes of transplanting them back into the garden next spring.  With the grow lights already in place and the plants right next to the kitchen where I will hopefully remember to water them, I thought I might try growing some lemon trees.

Ashley’s instructions looked ridiculously easy.  Basically, harvest the whole (no nicks or cuts) lemon seeds, wash them off, and immediately plant them ½” deep in pasteurized potting soil.  

That pasteurized potting soil presented a little bit of a hang-up.  I had to learn how, exactly, to pasteurize potting soil. 

Sure, I could take an hour and a half to run up to Home Depot and spend a small chunk of change.  But with all the disruptions to the supply chain and shipping issues, I thought I ought to learn.  Buying pasteurized potting soil may not even be an option in the future. 

So what’s the big deal about pasteurizing potting soil?  Can’t you just bring some dirt in from the garden?

Actually, you could try bringing soil in from outside.  I’ve done it before and been successful.  I also live in a very dry area.  I can’t recall ever experiencing any problems with plant diseases.  (Well, there was that one time, but it was on the tomato plants I bought at Grocery Outlet.  That disaster only made me want to grow all my plants from seed.)  However, with the interesting times we have in our future, it would be wise to take all precautions against plant diseases, and pasteurizing soil is really not at all difficult.

First off, the soil needs to reach a temperature of 180°F and maintain it for at least 30 minutes to kill off the harmful bacteria, viruses, fungi, insect eggs, and larvae.  If the temperature doesn’t get high enough for long enough, the pathogens may not be eliminated.  On the other hand, if the temperature goes too high, say above 200°F for too long, you risk destroying the structure of the soil.  That’s no good, either. 

Here are three ways to pasteurize your soil: 

Begin by scooping up all the soil you want to pasteurize.  Make sure you don’t have any fresh animal waste.  Just sayin’.

1.  Boiling water.  Place the soil in a metal or plastic bowl.  Pour boiling water over the top and get it thoroughly wet.  Cover the bowl with towels to keep it very warm—ideally at 180°F or a little higher—for at least 30 minutes.  Then drain the water or let it evaporate away. 

2.  Oven heat.  Place about 4 inches of moist soil in a metal bowl or deep pan.  Cover it with foil and put it in an oven set to 225°F.  Check the temperature at the center with an instant read thermometer every 30 minutes, and more frequently as the temperature approaches 180°F.  Once the soil temperature reaches 180°F, reduce the oven temperature to 180°F and leave the soil in the oven for 30 minutes.  Then remove the soil from the oven and let it cool to room temperature before planting. 

3.  In the summer, use a solar cooker of some sort.  It will take only a few hours to heat a black 3-gallon pot of soil enclosed in a plastic bag to 180 degrees.

References:

https://practicalselfreliance.com/growing-lemon-trees-from-seed/

https://extension.psu.edu/how-to-pasteurize-medium-and-sterilize-containers-and-tools