The Science of Composting

Composting involves a complex food chain of bacteria, fungi, mites, worms, beetles and other beneficial organisms that consume organic materials and produce humus. The composting process is mostly accomplished by bacteria, especially when it is proceeding rapidly at a high temperature. Bacteria quickly populate any good composting environment. Other organisms become active as the conditions become favorable to them. All you have to do is maintain the conditions that allow these organisms to thrive in your pile until the job is done. The proper environment for them is a mass of organic material that contains a balanced diet of nitrogen and adequate moisture and oxygen. For rapid composting, the materials should also have the right size particles and heat up to 120-150 degrees Fahrenheit.

A Balanced Diet of Carbon and Nitrogen

Like humans, compost organisms require a balanced diet of nutrients. You don't need to worry about proteins, vitamins, and minerals. Just balance the carbon and nitrogen in the composting materials by mixing high nitrogen green materials and high carbon brown materials
A compost pile made up of only brown materials breaks down slowly because it does not have enough nitrogen to support a large population of compost organisms.
A compost pile with too much green material begins to harbor pests and to lose its nitrogen in an ammonia form, which creates odors.
A good rule of thumb is to mix equal amounts of green and brown materials by weight or a larger volume of brown materials than green materials
Add brown and green materials in thin layers, then mix them
Where it is critical to prevent odor and pest problems, use more brown material.

Understanding Carbon and Nitrogen

Living things are largely made up of nitrogen and carbon. While alive, living things have a high nitrogen content they are "green". When they die and start decomposing and drying out in the open, airborne bacteria and other decomposers consume the protein rich tissues first and disperse the nitrogen in the form of dead bacteria, bug droppings, ammonia and other by-products. The carbon rich plant fibers stay intact much longer. This is the process that turns green material like grass clippings that are left in the open into brown materials like dry grass. Manure is considered a green material because it stays rich in nitrogen for so long.

Importance of Carbon and Nitrogen

Part of the purpose of composting materials in piles and bins is to keep their nitrogen from dispersing. Dead bacteria and the nitrogen rich materials that living bacteria excrete, including ammonia, diffuse and cascade slowly through the pile. New generations of bacteria and other composting organisms recycle this nitrogen as they consume the high carbon plant fibers for energy.

Adequate Moisture and Oxygen

Just as carbon and nitrogen must be balance, so does moisture and oxygen. Too much moisture leaves no air spaces, but too much ventilation dries out the materials. Early in the composting process, the main difficulty is to keep all the materials in a well-mixed pile moist. Later, as they break down in a crumbly, more absorbent, compact mass, the main challenge may be to keep the material aerated with fresh air. Rapid composting at high temperatures can deplete oxygen in the pile quickly, even early in the composting process.

Thoroughly moisten the material with a fine spray as you build your pile or add to it. A pile of dry organic material can shed water like a duck.

    If the pile is to dry:
  • Turn it, wetting the materials as they re-stack
  • Mix in damp materials such as food scraps or
  • Mix in fine particle, absorbent material like manure
If your pile is too wet, turn it and mix in coarse materials like dry leaves or straw into it. If the pile keeps drying out, help it retain its moisture and nutrients by building it larger, by putting it in a covered bin or by covering it with a tarp.

Importance of Moisture and Oxygen

Where moisture is lacking, decay is very slow. Where oxygen is lacking, materials are decomposed slowly by anaerobic organisms that can produce foul smells and noxious by-products. The best level of moisture a pile is as much moisture the materials can contain without filling the air spaces between the particles like wrung out sponge. When you squeeze the material, no water should drip out.

Most brown materials including leaves and wood chips are coarse and stiff and angular, creating air spaces in the pile and thus helping it stay aerated. Brown materials also tend to dry out quickly and absorb moisture slowly. It can be difficult to keep them moist. On the other hand, green materials tend to be moist and pliable and tend to mat together in an anaerobic mass.

Mixing green and browns together then also improves the balance between moisture and oxygen in a pile in most cases. Paper is an exception it is very "brown" but easy to wet and tends to stay matted together, so it helps to absorb excess moisture, but does not help with aeration.

The Right Particle Size

Composting happens where moist organic materials are exposed to air. When materials are broken down into small particles, there is more exposed surface area for composting organisms to attack. Microscopic organisms penetrate solid objects slowly.

    Break down large objects before adding them to a pile:
  • Twigs and leaves can be run over with a lawnmower or run through a leaf shredder.
  • Whole branches can be run through a chipper.
  • Garden plants or fleshy prunings can be chopped with a machete or pruning shears.
  • Food scraps can be cut up in the kitchen or chopped up in a bucket with a square-point shovel.

Importance of Particle Size

Rapid, aerobic composting occurs when green and brown materials are reduced to small pieces and thoroughly mixed together. That way every part of the pile gives decomposing organisms access to needed carbon, nitrogen, oxygen and water. Chunks of wood and other lumps of brown material are nitrogen-poor zones. Whole appels or other large pieces of green material are anaerobic zones. A pile full of large chunks of material will have too much air space and the surfaces will dry our rapidly. A pile of very fine materials such as sawdust and manure may have too little oxygen and require frequent turning.

A Healthy Temperature

The best way to know whether your compost is healthy is to take its temperature. Composting occurs most efficiently when the piles temperature rises to between 120 to 160 degrees Fahrenheit and stays there until most of the material has decomposed. Composting can be successful at much lower temperatures it just takes longer.

  • If your pile does not reach 120 degrees F, check to be sure the particle size is right, that the pile is big enough, and that is has the right balance of brown and green materials, moisture and oxygen. Building the pile larger, covering it with a tarp or burlap or putting it in one of the readily available compost bins helps it build up and retains heat.
  • If your pile gets too hot, turn it to release the extra heat and restore depleted oxygen. Adding water to dry materials as you turn them will reduce overheating and restore moisture.
  • Heat dissipates at the edges of the pile. There may be 8 inches or more at the outer edges and top of the pile that never reach 120 degrees, especially in open piles and wire bins. When turning the pile, scrape of these outer edges and work into the center of the pile to heat the material.
  • Composting thermometers with long probes are available at gardening centers.

Importance of Temperature

The body heat of teeming bacteria in a compost bin can build up temperatures of 160 degrees Fahrenheit or more when the materials are still fresh. As the material decomposes, it cannot support as many bacteria. As the number of bacteria decreases, the temp gradually drops. Temperatures above 130 degrees kill off most disease organisms within hours and temps above 140 degrees kill most weed seeds. However, rapid, hot composting depletes oxygen and necessitates frequent turning of the pile. Temps rising above 140 degrees kill off organisms that help with the later stages of composting, thus decaying the curing of the pile.