Monday, June 15, 2009


Household garbage often contains a large amount of organic debris that contains stored energy. One of the easiest and most satisfying ecological practices is to compost your waste and produce valuable humus, the organic-rich component of soil that is rich in nutrients and microbes and is essential for fertile soil. At our first house my wife and I bought a large plastic container for composting, but at our second house we used a more environmentally friendly and cheaper approach by building a compost container out of stakes and metal screens used for gardens (Fig. Compost_pile). If designed and maintained properly, compost bins do not usually smell badly, but to be safe we placed ours at the back of our yard. The disadvantage is that we have to walk a few hundred yards to dispose of waste in our compost pile, so we reduce the number of trips by using a small container that we fill and then carry to the compost pile. This is about the only effort required for passive composting, which takes about one year to completely breakdown organic debris into humus. Active composting can produce humus much more quickly, but requires much more effort, and in general I prefer the easy approach. We occasionally stir and water the pile, and then remove soil from the bottom of the pile for our gardens. And we follow some simple rules. First, we add no meat or fatty foods like butter that can attract animals and smell when they spoil. We try to use ½ green, wet material such as tree and bush trimmings and grass clippings that are nitrogen-rich, and ½ brown, dry material (decayed leaves, straw, wood chips) that is carbon-rich ([1], pp. 111-121), in addition to any compostable food waste we produce (banana and orange peels, used coffee grinds and tea leaves, eggshells, corn husks, artichoke leaves, and spoiled fruit and vegetables). We add these materials in layers. It’s better to have too much brown than green material, as too much green can cause formation of molds and bad smells. We don’t add weeds to our compost so as to avoid adding their seeds to our gardens when we add composted soil. Compost bins do not need sunlight, so we placed ours in a shady corner of our yard.

Start your compost pile by mixing together yard litter and foodstuffs, mixing in a small amount of soil that contains the necessary microorganisms, and adding a little water. Little may happen in the first few weeks, but once the microorganisms multiply and establish healthy colonies they will start digesting the waste, extracting energy for their metabolic processes and releasing some of the energy as heat. You will know that your compost pile is working when you feel it giving off heat. When oxygen is present the breakdown of organic matter can be described by the reverse of our model chemical reaction for photosynthesis:

C6H12O6 + 6O2 = 6 CO2 + 6 H2O

The heat comes from the energy of the sun, temporarily stored in organic molecules by plants utilizing photosynthesis. Essentially the same reaction occurs in our bodies when we consume food; respiration releases the energy stored in the food so our bodies can use it. Oxygen is present under aerobic conditions, and the microorganisms use it to breakdown (oxidize or combust) the organic molecules to extract their energy, but if the oxygen they use is not replaced, then eventually it will all be consumed, and under such anaerobic conditions the above reaction grinds to a halt. What happens next is that anaerobic fermentation reactions begin to breakdown the organic molecules and produce alcohol, the same process that we use to make bread and beer with yeast (the alcohol escapes from the bread during cooking). Anaerobic respiration also produces lactic acid in our muscles when we strenuously exercise: the body cannot replace the oxygen fast enough, so it begins to break down sugars and fats anaerobically ( The problem with alcohol production in the compost pile, however, is that alcohol is a disinfectant, so it sterilizes the pile, wiping out the microbial communities. And anaerobic respiration tends to produce odors from compounds like hydrogen sulfide, which gives the “rotten egg” smell you associate with swamps, where it is produced in the same way.

Finished compost should be dark brown. If it is black, your compost pile does not have enough oxygen; you need to add less water, and aerate the pile by turning it over. A simple approach to solve both of these problems is to place perforated PVC pipes, ones that are slightly greater in length than the diameter of your pile, at various heights in the pile. The pipes will suck air in to provide oxygen to aid decomposition, and drain off excess water.

Creating your own soil by composting is one more way to move yourself toward sustainability and independence [1]. And composting, combined with recycling, has greatly reduced the amount of waste we put in garbage cans.

1. Kellogg, S. and S. Pettigrew, Toolbox for Sustainable City Living. 2008, Cambridge, MA: South End Press. 241

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