Gardeners have long made and used compost because of the way it improves garden soil. Home composting transforms yard debris and food scraps into a valuable soil amendment and closes the recycling loop in our own backyards (Figure 1). Today, many cities have municipal composting programs. These programs include curbside yard and food debris collection, large-scale composting at commercial facilities, and resale of the finished compost to gardeners and landscapers. Nonetheless, backyard composting remains popular in areas without municipal programs, and for gardeners who want to make and use their own compost.
The Science of Composting
The Cycle of Growth and Decay
Composting carries out part of the earth’s biological cycle of growth and decay. Plants grow by capturing the sun’s energy along with carbon dioxide from the air and nutrients and water from the soil. When plants (and the animals that eat them) die, they become raw materials for the decay process. Bacteria, fungi, insects, worms, mites, and other creatures convert some of the carbon from dead plants into energy for their own metabolism, releasing carbon dioxide into the air. They also cycle some of the carbon and nutrients from the decaying plants into their own bodies as they grow, and eventually back into the soil, where the cycle begins again. The material that remains from the decay process is similar to soil organic matter. It holds water and nutrients in the soil and makes the soil more porous and easier to dig.
Fast or Hot Composting
We can manipulate the decay process to make it proceed faster. We do this by balancing food, water, and air in the compost pile to favor the growth of thermophilic (high-temperature) microorganisms. One byproduct of microbial activity is heat. When conditions are favorable for high-temperature microorganisms, compost piles heat rapidly to 130°F–150°F. This temperature range kills most weed seeds and pathogens (disease organisms) (Zabroski 2015). Once the hot phase is complete, lower temperature bacteria along with fungi, worms, insects, and other organisms complete the decay process.
If we do not maintain ideal conditions for hot composting, microorganisms will still break down the wastes, similar to what happens in soil. Decay will be slower, cooler, and less effective at killing weed seeds and pathogens.
Managing the Decay Process
You can affect the speed of the composting process and the quality of the compost product by managing the factors described below.
Food (Raw Materials)
For fast composting, the initial mix must have the proper moisture and air content as well as organic materials that provide a rich energy and nutrient source for bacteria. Table 1 and Figure 2 show some materials commonly used in making compost. They are separated into “energy” materials, “bulking agents,” and “balanced” materials.
Energy materials provide the nitrogen and high-energy carbon compounds needed for fast microbial growth. If piled without bulking agents, these materials usually are too wet and dense to allow much air into the compost pile. The pile will become anaerobic (lacking oxygen) and produce foul, rotten odors.
Bulking agents are dry, porous materials that help aerate the compost pile. They are too low in moisture and nutrients to decay quickly on their own.