The beauty of composting is that it’s a process that bridges the inanimate world with the animate. Composting takes dead organic material, such as lawn clippings or food scraps, and through the metabolic actions of small organisms, turns it into a substance rich in the nitrogen, phosphorus and potassium that support plant life. Without the work of tiny biotic agents, compost wouldn’t exist.
Insects play an important role early in the decomposition of the material that will become compost. Ants, sow bugs, beetle grubs and springtails all eat decaying plant and fungal matter. As this matter passes through their digestive systems, it gets broken into smaller pieces that can be refined later in the composting process. Additionally, as ants travel through their underground tunnels, they carry phosphorus, potassium and other mineral nutrients to the compost pile.
Other small members of the animal kingdom that lack spines and so are called invertebrates also aid in composting. Several invertebrates process organic material from sources other than plant matter for the compost pile: Millipedes digest and metabolize insect carcasses, for example, while centipedes consume other living invertebrates. Most important, though, may be the earthworms, which excrete waste, called castings, that is richer in essential plant nutrients than most soil.
With the exception of earthworm castings, much of the waste excreted by invertebrates undergoes further metabolic processes and releases more nutrients. Bacteria are integral to this stage of the composting process. They work with or without oxygen to metabolize the carbon-based compounds that make up the bulk of organic matter and are still present in invertebrate waste, producing new compounds that plants can use to grow.
After a wide range of less complex bacteria have metabolized most of the organic matter in the compound pile, a more organized type of bacteria, known as the actinomycetes, may flourish. Actinomycetes, once thought to be fungi, appear as thin gray filaments throughout the compost pile. They are essential to the decomposition of materials like starches and proteins that contain less carbon than the materials that lower-form bacteria process.
Fungal spores come to the compost pile on the wind or on the bodies of invertebrates. Because fungi prefer cooler temperatures, they are most likely to develop in compost after the bacterial action has subsided. Like actinomycetes, they break down the materials that basic bacteria have difficulty processing; the fungi are partial to the cellulose and lignin in plant matter. The appearance of fungi is usually a sign that a compost pile is ready for garden use.