Is there a certain soil type best for composting? The soil near my home is mainly sand/clay (Arizona's high desert, Four Corners region). How can I best help the composting process taking into account the amount of sun, wind and water received in my region? What is the most sustainable way to do so?
Asked by Dianne Nakai, '03, Window Rock, Ariz.
At some point in your life, you've probably seen a banana peel on the ground. If you passed by that banana peel every day for several weeks . . . well, you probably should have picked it up. But if you didn't, you might have noticed that each day it became browner and looked less like a banana peel as it rotted there on the sidewalk. At least as early as the Roman Empire, people realized that they could speed up this natural decay process for their benefit. That sped up process is composting.
How does compost work?
Let's take a look at the process of food decay. Decay occurs thanks to two different kinds of creatures: macroorganisms and microorganisms. Macroorganisms (larger bugs, earthworms and other creatures) are responsible for the physical break down of those banana peels and grass clippings in your compost pile—chewing up the material, leaving it in smaller pieces, and increasing its surface area.
Microorganisms (small bacteria and fungi) are highly abundant but nearly invisible. They are so small that it would take 25,000 of them laid out to reach just one inch. Since they can eat almost anything, microorganisms are indispensible to the composting process. They eat the food and yard waste in the compost pile and it chemically breaks down inside them. When they excrete the waste, it is in the form of nutrients, like nitrogen and phosphorous. Those nutrients are food for plants, so adding compost to the soil allows the plants to grow and survive.
Another benefit of compost, especially in deserts, is its effect on soil structure. Soils that have very fine particles, such as clay, tend to become compacted very easily, which is bad for water drainage. When soil is compacted, water is unable to flow through it, making it difficult for plants to absorb any water through their roots. Compost in clay soils causes the particles to separate out, increasing the amount of air and water space in the soil. Soils with larger particles, such as sand, face the opposite problem. There is so much space between particles that water and nutrients flow through too fast for plants to access them. Adding compost to sandy soils causes the particles to clump together, increasing the amount of water and oxygen the soil holds.
Keeping your compost pile happy
Though microorganisms are not picky eaters, their size and simplicity means they are sensitive to environmental conditions. They are susceptible to changes in oxygen, moisture, temperature and nutrient ratio. These factors are important to monitor in your compost pile no matter where you live so that the microorganisms don't become inactive or die. Moisture and temperature were already discussed in the Essential Answer; here we'll look at oxygen and nutrient ratio in more detail. The techniques you use to monitor them are the same in any locale.
Microorganisms, like humans, need oxygen to produce energy and grow. If they run out of oxygen, they suffocate, and decomposing stops. The most common way to add oxygen to a compost pile is to physically turn it over with a shovel. Regularly turning the pile over fluffs it up and lets fresh air enter. There are also compost aeration tools available at gardening stores to do this job.
Microorganisms will eat most food or yard waste, but they do have certain nutrient ratio preferences that enable them to best grow and reproduce. The two important nutrients for microorganisms are carbon, often called "brown material" because it is found in drier, browner materials like wood chips; and nitrogen, often called "green material" because it is found in fresher, greener materials like grass clippings. The microorganisms survive best when the carbon-to-nitrogen ratio is 25 or 30 to 1, but getting an exact ratio is not as important as simply mixing up brown and green materials. A good estimate is to have a green-to-brown volume ratio between 2 to 1 and 4 to 1.
Municipal Solid Waste Discarded in 2009. Graphic: Courtesy U.S. EPA
The big picture
If you are not the gardening type, it may be daunting to consider keeping a compost pile. They require a sizable amount of attention and effort. If left unattended, they can become smelly, and if not placed somewhere discreet, they can be an eyesore. From an environmental standpoint, however, composting is one of the purest and most useful forms of recycling. It returns nutrients to the earth to be used again for plant growth. It also keeps reusable materials out of the landfill. In the United States, food and yard scraps made up 27.8 percent of waste in 2009. Food scraps alone were the largest contributor to waste, weighing in at 33.44 million tons. Even though this waste is decomposable, when it is added to the landfill it does not decompose in a beneficial way. Instead, it rots and becomes a source of a very strong greenhouse gas called methane, which doesn't help our climate change problem. Composting, then, helps reduce greenhouse gas emissions.
If you just can't fit composting into a busy schedule, you can still keep your food scraps from reaching the landfill. Municipal composting programs are gaining popularity in many cities and counties. You can search online for one near your hometown. Many of these programs even make it easy for you by picking up decomposable waste from your home. Another option is to buy less food in the first place so that less of it gets wasted. The Environmental Protection Agency reports that American households throw away an average of 470 pounds of food, or 600 dollars, every year. If there is anything that is going to get us to reduce and reuse food waste, it's money. Hopefully, the combination of these financial incentives and the satisfaction of giving back to a continuously providing Earth will inspire all of us to hop on the composting train, regardless of where we live.
Gretchen O'Henley received her bachelor's degree in Earth Systems in 2011.
