Chemistry & Composting

Household waste and yard trash combines with chemistry in a compost pile.

Composting is taking household organic waste like vegetable and fruit peels and scraps, grass clippings, wood chips, shredded paper and plant cuttings and letting them decompose. After the process is finished, the resulting mix makes a very good fertilizer for plants and gardens. While the biggest components of composting are time and materials, knowledge of the chemistry behind composting allows composters to help the process occur more quickly and to yield better quality results.

Carbon and Nitrogen

Decomposition relies on many different elements, but the most important ones are nitrogen and carbon. The microbes that eat the material and perform the decomposing are composed of mostly carbon. They also use carbon for fuel. Nitrogen supplies the building blocks for cell growth and function, so it is a crucial part of a microbes' diet as well. There needs to be a good ratio of carbon to nitrogen for the microbes to thrive. According to Cornell University experts, that ratio is 30:1, or 30 parts of carbon for each part of nitrogen. Larger concentrations of nitrogen produce ammonia and contribute to odors. Larger concentrations of carbon do not allow for maximum microbial growth and slow the decomposition process.

Element Ratios

Certain waste materials are high in carbon, while others are high in nitrogen. By using a chart to discover the amount of each element in different compost ingredients, you can create the proper mix. In general, green wastes or moist, fresh cut items such as grass clippings, plant cuttings and fruit and vegetable scraps are high in nitrogen. Wood products or dry materials such as shredded paper, wood shavings, sawdust and fall leaves are rich in carbon. You can use a chart to determine the exact amounts for each item in a compost pile, or try to keep a 30:1 ratio of carbon rich items to nitrogen rich items. Getting donations of paper, fall leaves and tree parts from neighbors and businesses can aid in maintaining the right ratios.

Oxygen

Another crucial element for a compost pile is a good supply of oxygen. Microbes use oxygen to consume carbon and it combines with carbon that is released during consumption to produce carbon dioxide. Using an air blower on the pile or regular turning of the pile will keep a healthy supply of oxygen throughout the pile. Oxygen also prevents the production of foul smelling gasses such as hydrogen sulfide.

Heat

Fresh air maintains the right oxygen levels and keeps the temperatures in a healthy range for microbes. All of the chemical activity within the compost pile produces a lot of heat. According to the University of Washington, temperatures inside a compost pile can reach over 170 degrees Fahrenheit. Once the temperature in the pile reaches over 160 degrees, microbes slow down and begin dying. Temperatures of between 70 and 90 degrees are optimal for a majority of microbes, though some thrive at temperatures above and below that range.

Result

As the microbes eat the compost material, they release about two-thirds of the carbon, so in time, the ratio changes within the compost material. The end product has a carbon to nitrogen ratio between 10:1 and 15:1. This higher nitrogen ratio is what makes compost so beneficial as a fertilizer, since it adds nitrogen to the soil.

References

• Cornell University: Compost Chemistry
• Washington State University: Compost Fundamentals
• Kansas State University Extension: Composting