Anaerobic Composting
An Ancient Fertility Secret?


What is that horrible odor?

Plug your nose!

Anaerobic composting odors - only a true compost junkie could come to appreciate them. The prior statements are some of the most common ones made by individuals when they first smell the products of compost made without oxygen (i.e. anaerobically); however, before you go running for the hills, do know this...

For centuries, Chinese rice farmers have used anaerobic composting to produce the majority of the nutrients, and soil fertility, for their rice paddies. Prior to the late 1900s, it was a common practice for many Chinese rice farmers to ferment a mix of pond/canal sediments, leguminous green manures (e.g. vetch), rice straw, and animal manures in water-filled pits, and then spread the fermented products on their paddies. The product of their anaerobic composting was called "oufei", or "water-logged compost".

Could the lost art of developing these fermented compost products offer a new means of fertility for our current gardens and crop growing?

Let's find out...

What is Anaerobic Composting?

It is the biological breakdown of organic materials by living anaerobic organisms (e.g. bacteria). As the word anaerobic implies, these organisms thrive in environments in the complete absence of oxygen. This differs dramatically from most of the other discussions we have been having on this site. Usually, we discuss the need for lots of oxygen within a compost pile; however, it must be noted that you can still obtain a breakdown of compost ingredients even in oxygen poor piles.

This type of composting, or organic matter breakdown, is typical of the processes found in marshes, and bogs that produce peat moss.

If you compost using the static method (you don't turn/aerate your pile), there is a very high likelihood that your pile goes through aerobic, and anaerobic, cycles while it is breaking down. For instance, when your pile is first built, it will contain lots of oxygen, and aerobic bacteria and microbes will proliferate; however, as these microbes increase in number, they use up a lot of oxygen in your pile. As the oxygen levels in your pile begin to drop, the conditions start to favor anaerobic bacteria. Eventually, these conditions will reverse again before your compost is completely cured.

How to Anaerobic Compost

There are a couple different methods to create compost anaerobically, including:

1. Regular Static Compost Piles

Assemble your compost pile as per usual (for more detailed composting instructions, please visit our how to compost page), except be sure to add more water. Ideally, you'd like your pile to have upwards of 70% moisture to encourage anaerobic conditions. By adding more water than usual, you are driving out the oxygen from your pile.

Once built and moistened, you'll want to make sure you cover it, and monitor its moisture levels closely. When your ingredients become slimy, you'll know you're on the right track. Be prepared for some unpleasant odors.

2. Garbage Bag Compost

For complete instructions on composting anaerobically using a garbage bag, please see our indoor composting page. This style of anaerobic digestion is much the same as the breakdown that occurs inside silage bags. You may have seen these huge white plastic bags lying in farmer's fields.

3. Submersion Compost

This technique mimics the composting methods used by the Chinese rice farmers mentioned in our introduction.

To make submersion compost, you'll want to place all of your composting ingredients into a large vat or container, fill it with water, and allow it to breakdown. This method helps to minimize the production of odors because the odor-causing gases are exchanged with the water molecules prior to evaporating.

4. Anaerobic Bucket Compost

One of the easiest, and least odor-emitting, methods of producing anaerobic compost is to use a simple bucket. Start with a 5-gallon bucket; cut off the bottom; bury it a couple inches in the soil; fill it with organic waste (e.g. food scraps, manure); and cover it with a lid. Allow this covered bucket to sit undisturbed for one year.

When you return, you'll open the bucket and find the ingredients have all turned into usable humus. It is essential that you trust the ingredients in your bucket are breaking down. Avoid the temptation to open the lid and check on your compost; this will introduce oxygen and halt the entire breakdown process.

Advantages of Anaerobic Composting

  • Requires a lot less work than aerobic composting
  • Produces more usable humus per volume of original composting ingredients compared to aerobic composting
  • Finished product contains higher levels of ammonium, since anaerobic microbes tend to use less nitrogen than aerobic microbes
  • Process of composting anaerobically produces biogas (e.g. methane and carbon dioxide), which can be captured and used for energy/fuel

Disadvantages of Anaerobic Composting

  • Putrification of organic materials can lead to very unpleasant odors, due to release of sulfur-containing compounds such as hydrogen sulfide
  • Process requires a lot more time to produce usable compost compared to aerobic composting
  • If anaerobic compost is not allowed adequate time to breakdown (e.g. one year), there are risks that the compost will contain pathogens. Since there is not enough heat generated in an anaerobic pile, you must rely on your microbes. That is, if your compost is given enough time to breakdown, any pathogens that may have been present will eventually be out-competed by more beneficial microbes.

    Here is an article from Washington State University that discusses the natural destruction of Ascaris eggs (produce intestinal parasites) in an anaerobic composting system.

Our Conclusions

Overall, we think everyone should experiment with anaerobic composting. It is so easy. Start by using one of the four techniques mentioned above and see what results you get. If you're happy with the results, try adding a handful of composting worms to the finished product, and allow them to further enrich it with their wonderful worm castings.

If the rice farmers of China have been using anaerobic techniques for many centuries, there is definitely merit to the concept. It could very well be an ancient composting secret that has the power to further enrich all of your growing experiences.