From the outside, a compost pile might look simple and fairly boring. But inside, fascinating biochemical processes are unfolding. You don’t need to know chemistry to make compost, but having a basic understanding of what’s going on in its interior will help you choose the optimum conditions for best results.
Bacteria and fungi do most of the work in your compost pile. These organisms need carbon for energy and a variety of nutrients; of these, nitrogen is by far the most important. All fungi and most bacteria also need oxygen. Some bacteria can live and grow without oxygen; these anaerobic microbes tend to release smelly compounds like hydrogen sulfide as a byproduct of their metabolism. So, if your compost pile gives off a foul odor, it is poorly aerated, and the anaerobic bacteria are taking over.
It’s important to keep compost well-aerated because aerobic (oxygen-loving) bacteria are by far more desirable. Unlike anaerobic bacteria, they don’t create a foul stench, and they break compost down much more rapidly and completely, leaving valuable nutrients like nitrogen, phosphorus and magnesium in the decomposed material.
Compost acidity or alkalinity (pH) is another important factor. Initially, the pH of the compost tends to drop as bacteria release organic acids. Later on, these acids will be broken down more completely, and the pH will stabilize. If oxygen is absent, however, pH can drop to 4.5 or so. This strongly acidic condition kills many of the more desirable bacteria and slows the rate of decomposition.
Decomposition in a compost pile goes through several distinct phases. At first, the psychrophilic (cold-loving) bacteria predominate. As they break down organic compounds, they release heat and increase the temperature of the compost to above 70 degrees Fahrenheit. Next, mesophilic (moderate-temperature-loving) bacteria take over.
The temperature generally continues to rise until it passes 100 degrees Fahrenheit, at which point the mesophilic bacteria give way to thermophilic (heat-loving) bacteria. The reign of the thermophiles typically lasts a few days at most. As they use up the available carbon and nutrients, the thermophiles die, and the mesophilic bacteria take over again. Finally, the fungi become more active and take the lead during the final stages of decomposition.
The ratio of carbon to nitrogen in a compost heap is the last important factor to consider. A 30:1 ratio by weight is considered optimal. The hungry microbes will ultimately convert much of the carbon into CO2. The nitrogen, by contrast, will become incorporated into microbial proteins and DNA. When the microbes die, they’ll leave the nitrogen behind for your plants to use.
Paper, bark and wood chips all have C:N ratios of 100:1 or more; autumn leaves and straw are also relatively low in nitrogen. However, vegetables, coffee grounds, grass clippings and manure, have C:N ratios of 20:1 or below. Mixing nitrogen-poor and nitrogen-rich ingredients is a good way to keep the C:N ratio in the right range.