Medium for Compost Organism Culture

A few days ago I was discussing my upcoming book on composting with a friend, and he asked a very worthwhile question: since you can’t actually see what is going on in a compost pile, how do you know?

The answer is that I have a laboratory and I have extensively cultured the organisms in compost and differentiated their populations by incubating at various temperatures and so forth. I can’t afford to equip a modern laboratory, but you may be surprised to discover that with just a few minor variations, the equipment and techniques of 100 years ago work just as well today and at trifling cost accessible to anyone.

For a very good introduction to all of the techniques necessary for sterile culture, separating organisms, and differentiating them through medium, incubation, growth habit, staining and more, you can refer to the free e-book on the Project Gutenberg website entitled The Elements of Bacteriological Technique. The information in this book is 100 years old and some of it is certainly out of date. So you should already have modern high school biology under your belt to correct errors.

Thankfully, with very little change, a lot of what is in that book is still in use and remains useful today. The materials used 100 years ago are still used today and can be purchased over the Internet. And, like I said, with a bit of creativity you can outfit a respectable and useful laboratory for hundreds of dollars rather than thousands or tens of thousands. A big bonus is that modern stuff is of higher quality due to improvements in manufacturing. Some of my gear is pretty non-standard. For example, my incubator is a drink cooler equipped with a Peltier junction and controlled via an Arduino. But bacteria don’t care how fancy your gear looks.

Anyway, to the nuts and bolts. My friend wanted to know what I use as a medium for growing bacteria from compost.

The medium for culturing bacteria should be as close as possible to the natural environment. What I do is pressure can compost tea made with distilled water in quart jars to keep it handy. I pressure can it for 30 minutes, more than assuring the death of all organisms, but retaining the other elemental constituents. When I am ready to make my medium, I use this sterile compost tea as a base, and incorporate:

  • 20 grams of agar
  • 10 grams of dextrose (I use the stuff from a home brewing store instead of the $pricey$ stuff from the lab company)
  • 500 mg of Ammonium Phosphate (Again, from the home brewing store)
  • 200 mg of Magnesium Sulfate (Epsom salts from the grocery store)
  • 500 mg of Peptone (from the lab supply company)

There is a special trick to incorporating the agar. Agar is a seaweed product that will turn the medium into something like gelatin. Mix it into a slurry with one cup of the compost tea, then add the slurry into the rest of the medium.

After this, bring the whole mixture to a boil over a water bath while stirring. This will let the agar dissolve completely and sterilize it. I then filter it through several layers of cheesecloth while hot and use it in my Petri dishes.

This is what I use for examining aerobic compost organisms and facultative anaerobes.

But, as you can see, there is nothing here that can’t be done by anyone familiar with canning. Science is not mysterious!