Composting

What Is Composting?

Composting is a biological process that decomposes organic materials into a high-quality soil amendment. This process occurs in nature but can be accelerated and improved under certain conditions.

The finished compost beneath this mesh bin is dark and crumbly.
Tufts University

What Are the Benefits of Composting?

When added to soil, compost breaks up heavy clay soils, helps sandy soils retain water and nutrients, and releases essential nutrients for plant growth. Compost also contains beneficial microscopic organisms that build up the soil and make nutrients available to plants. In addition to the soil-related benefits, composting saves landfill space by diverting food and yard waste that would otherwise be discarded.

What Can I Compost?

Most plant material can be composted. Organic trimmings in the landscape, such as fallen leaves, pine needles, grass clippings, flowers, and the remains of garden plants, make excellent compost ingredients. Woody yard trimmings can also be composted. Kitchen scraps, such as fruit and vegetable peels, crushed eggshells, tea bags, coffee grounds and filters, are also compostable.

What Materials Should Not Be Composted?

When it comes to composting, certain materials that may create foul smells, attract pests, or introduce pathogens should be avoided. Do not compost meat, dairy products, oils/grease, or animal waste should not be composted. Although some animal manures may be composted, the process must be managed properly. To minimize potential environmental and health risks, it is not recommended to add animal manure to a home compost pile.

Weeds that have not gone to seed can be added to the compost pile. However, weeds with large storage roots, such as nutsedge (Cyperus spp.), Florida betony (Stachys floridana), or greenbrier (Smilax spp.), should be allowed to dry in the sun before composting to reduce their chances of survival.

The heat produced in the center of a compost pile can kill many pests, including weed seeds and pathogens. However, the temperature needed, and the duration of that temperature can vary. For this reason, it is best to discard weed seeds and diseased plant material instead of composting.

Carbon To Nitrogen Ratio (C:N)

Carbon and nitrogen are the most critical elements needed for microbial decomposition. The ideal carbon-to-nitrogen ratio (C:N) is 30 parts carbon to 1 part nitrogen by weight, which is written 30:1. Undesirable odors tend to be a problem when the ratio is lower than 30:1 because excess nitrogen is released as ammonia gas. If the ratio is higher than 30:1, decomposition is slow due to insufficient nitrogen.

Materials containing a large amount of carbon are known as browns. Woody yard trimming, fall leaves, and shredded paper are examples of brown materials. Green materials, such as vegetable scraps, coffee grounds, and grass clippings, are high in nitrogen and typically moist. Table 1 provides the average carbon-to-nitrogen ratio for some commonly composted materials.

Table 1. Average Carbon to Nitrogen Ratios for Organic Materials

Surface Area & Size of the Compost Pile: Since decomposition will happen more quickly for smaller pieces, it is best to chop or shred materials before adding them to the compost pile.

The overall size of a compost pile is important. Piles should be at least 3 cubic feet (3 feet x 3 feet x 3 feet) and no larger than 5 cubic feet (5 feet x 5 feet x 5 feet). Piles less than 3 cubic feet have trouble maintaining heat, while piles larger than 5 cubic feet do not receive enough air movement. Turning a large pile is also laborious.

Moisture & Aeration: Decomposition will be reduced, and unpleasant odors may be produced if there is too little or too much moisture in a compost pile. The compost should be moist but not soggy, similar to a wrung-out sponge or damp rag.

The microbes doing the work in a compost pile require oxygen. This can be achieved by turning the material with a digging fork or shovel once a week. If turning isn’t possible, oxygen can be introduced by poking the pile with the handle of a garden implement. An unmixed pile will take longer to decompose into finished compost.

Temperature & Time: Heat is generated due to the microbial decomposition process. Most harmful pathogens, as well as weed seeds, are destroyed when the temperature is over 140°F. A long-stemmed compost thermometer can be used to check the interior temperature of the pile at least 12 inches from the surface. The intensity of the process depends on the amount of nitrogen in the materials. The time required to produce compost depends on the kind and coarseness of the materials, pile volume, and availability of moisture and air.

Some Composting Methods

Fast Composting: This method, also called hot composting, is labor intensive but produces the fastest results. Gardeners utilizing this method focus on the ratio of browns to greens, particle size, moisture, aeration, and temperature.

Slow Composting: In slow composting, materials are added as they become available. The rate of decomposition is decreased since less attention is devoted to the process.

A commercially available single chamber compost bin is shown with finished compost at the bottom.
Photo Credit: Hopsalka, Adobe Stock

Composting Structures

Composting structures can be made from a wide variety of materials or purchased commercially. Each has pros and cons, and the user should select the system that best meets their needs. In general, the following should be taken into consideration when selecting a compost structure:

• Adequate volume to generate heat.
• Access to add and remove material.
• Built-in ventilation or access to aerate.

A lid can exclude excess moisture and limit access to scavengers.
Karen Russ, ©2007 HGIC, Clemson Extension

Single Chamber: Single chamber bins can be constructed from a variety of old materials, such as wooden pallets or wire mesh supported by a wood frame. Single chamber bins are also available commercially. Many of these are designed as slow composting systems in which brown and green materials are added in thin layers from the top, and the finished product is harvested from the bottom.

Three-Chambered Unit: A three-chambered bin works on the assembly line principle and is an efficient structure for fast composting. Two bins are used for composting, while the third bin is used for turning. A lid can shield the pile from rain, allowing the user to control the moisture level. A lid can also limit access to scavengers, although this generally isn’t a problem as long as vegetable material is covered.

Tumbler: A tumbler is a single bin that has a mechanism for rotation, aiding in aeration. Tumblers can be constructed at home or purchased commercially. Material may slide rather than mix when turning a tumbler. Interior baffles can help combat this problem.

Compost tumblers have a rotation mechanism that aids aeration.
Karen Russ, ©2007 HGIC, Clemson Extension

Using Compost

Finished compost is dark brown or black, smells earthy, and no longer resembles the parent material. If there are some chunks of larger, partially decomposed material, a screen can be used to remove them.

Although compost contains nutrients, it is a soil amendment rather than a fertilizer. Compost can be incorporated into garden beds or used to create a soil-based potting mix containing equal parts compost, topsoil, and sand.

Troubleshooting

Composting is relatively easy, especially if utilizing the slow composting process. Table 2 illustrates a few common problems and potential solutions.

Table 2. Compost Troubleshooting Guide

Originally published 06/99