Long before our modern-day waste management techniques were invented, nature relied on a simple solution to turn organic food scraps into nutrient-rich soil. Composting, the process of converting organic materials into soil through natural decomposition, is an effective method for turning your waste into something useful. There are four essential components to understand about the science of composting: the three thermal phases, greens-to-brown ratio, oxygen content, and water content.
Starting off with temperature, “thermophilic” composting helps provide the correct climate to increase the speed of decomposition and kill organisms that may be detrimental to the process. There are three phases — a mesophilic initial phase, a thermophilic phase, and a mesophilic maturation phase. The first phase includes temperatures of ~50 – 104ºF, where there is a rapid increase in bacteria and fungi over the first 3 days. They utilize the available simple sugars to proliferate quickly, forming a community of helpful microorganisms. The thermophilic phase comes next and lasts around 2 weeks. Here, the temperature exceeds 104ºF, and provides room for thermophilic microorganisms to break down more complex molecules, such as proteins. Finally, the temperature of the pile decreases and it enters its third phase, mesophilic maturation. This is the longest phase by far and can take several months. Stubborn compounds, such as lignin, are degraded here, and the compost ends up in its final state – a pile of fine, nutrient-rich soil (Cornell).
While this might seem complex, it is easy to get started composting right in your own backyard! The first step is to differentiate between “greens”, nitrogen-rich materials, and “browns”, carbon-rich materials. Greens include organic materials such as vegetable scraps, coffee grounds, and more. Browns are plant stalks, shredded paper, and woodchips. It is essential to start a compost pile with a six-inch layer of browns, then layer green and brown material in alternating levels (US EPA). Maintaining a 3:1 volume ratio of browns to greens preserves the correct carbon-to-nitrogen ratio. Nitrogen is necessary for bacteria to grow proteins, however, too much can create an excess of ammonia. Likewise, microorganisms use carbon as an energy source, but an excess can slow down composting (Cornell).
The final two pieces of the puzzle are oxygen and water content. Controlling the amount of oxygen in the pile is essential to ensuring consistent aerobic decomposition. Aeration expels trapped heat, water vapor, and other noxious gases (NRDC). Anaerobic respiration, or respiration by bacteria that do not need oxygen, can occur when oxygen is lacking. This is problematic for the compost because it can create unpleasant odors and standing piles of water. Therefore, it is beneficial to turn your compost pile about twice a week. Moisture is needed to promote microbial growth, however, too much can cause the unwanted anaerobic respiration (Wisconsin). In the Bay Area’s Mediterranean climate, it is necessary to add water during the warm, hot summer months, and ensure the pile is shielded from the elements during winter months (CalRecycle).
The composting process (Rynk, 1992)
Overall, composting is an excellent way to decrease the amount of food waste from your home that will end up in a landfill. To get started, you can obtain a bin, store your browns & greens, layer your pile in the bin, and then maintain it through regular aeration and watering. By understanding the three thermal phases, the correct greens-to-brown ratio, and the amount of oxygen and water needed, the science of composting becomes clear and accessible. This rewarding process is easy to get started and can transform your kitchen scraps into fuel for your garden!
By Charu Vijay
O2I Volunteer