fbpx Composting | The Morton Arboretum


Composting is a biological process in which plant material, such as leaves and grass clippings, and other sources of organic matter decompose into humus – a dark brown, crumbly mass resembling rich garden soil. The resulting compost can be applied as a soil amendment, top-dressing, or mulch. Autumn, when many fallen leaves are available, is a handy time to start a compost pile, although it can be done any time of year.

When we make compost, we are doing exactly what nature does all the time by recycling valuable nutrients for reuse. The work of decomposition is done by bacteria, fungi, and other microorganisms, helped by earthworms, insects and a variety of small creatures. For the home gardener, composting offers not only a way to improve soil but the opportunity to recycle yard and kitchen wastes while avoiding the expense and effort of disposing of these materials. 

Wooden fence around a compost pile

Why compost?

Incorporating organic matter, or humus, in garden and landscape soils is a necessity often overlooked by gardeners. Organic matter helps maintain a steady supply of plant nutrients, especially nitrogen and potassium. It also improves the soil’s ability to absorb rainfall or irrigation water and to reduce surface runoff, yet holds nutrients loosely at the optimum, slightly acidic pH so they’re easily released into the soil to meet plant needs. Adding compost to soil increases earthworms and soil microbial activity that benefit plant growth.

What materials can be composted?

Most of the ingredients for the compost pile are raw materials from the garden, grass clippings, sod, leaves, hedge clippings, weeds, and discarded plants. Kitchen scraps such as fruit and vegetable trimmings can also be added to the pile. Do not include grease, fat, meat or bones because they are slow to decompose, will cause odors, and attract rodents. Small twigs and branches of trees or shrubs should be cut up before adding to a compost pile, to speed up  decomposition.

Sewage sludge and animal waste is not recommended for use in compost, because it may transmit certain diseases and contain toxic amounts of heavy metals such as cadmium and lead. Also, diseased plant material or weeds that have gone to seed should be avoided.

Do you need a compost bin?

Usually the compost pile is contained in some type of enclosure, although it can be made without one. There are commercially available compost bins made of slatted metal, plastic or plasticized wire mesh. Perforated trash cans may also serve the purpose, or simple containers can be constructed of galvanized wire fencing, boards, bricks or concrete blocks. It is important that there be enough openings to allow adequate air movement.

The compost pile should be located in a visually unobtrusive area and where odors from possible anaerobic decomposition would not be a nuisance. A properly functioning compost pile has no unpleasant odor, but you can get a smell going by dumping on a layer of matted, wet grass clippings that doesn’t break down properly.

When compost forms, heat is produced. If the pile's internal temperature reaches 150 to 170 degrees, the heat will kill weed seeds, insect eggs, and disease organisms. In order to reach that temperature, the pile should have a minimum volume of 1 cubic yard, which is 3 feet by 3 feet by 3 feet. If it is much larger, the air necessary for decomposition will not enter the pile and it will need to be turned frequently for the center of the pile to decompose sufficiently

A compost pile built with alternate layers of plant material and garden soil.

Creating a compost pile

Initially, the compost should be constructed in layers. Alternate moist and green materials such as grass clippings or kitchen wastes with dry materials such as leaves or cornstalks.

A pile built primarily of dry materials will decompose very slowly, because it lacks nitrogen and water. Compost that is made primarily of fresh, green materials will decompose very rapidly, but will shrink as water is lost, leaving only a small volume of compost. Therefore, combining equal volumes of dry to moist is optimum. If the organic materials seem dry, enough water to promote decomposition should be added.

Chopping the materials into smaller pieces is not necessary, but doing so will greatly increase the rate of decomposition because the microorganisms have more surface area to work on. This is especially true for coarse, dry ingredients.

Another method of constructing a compost pile, perhaps the one most commonly used, is to alternate layers of plant material and garden soil. Begin by placing a 6 to 8-inch layer of organic matter inside the composting container. The microbes needed for decomposition are already present in this organic matter. Adding a 1-inch layer of garden soil, which contains many more microorganisms, will speed the process. Continue building the pile with alternate layers of soil and organic matter as they accumulate throughout the growing season.

Maintaining a compost pile

The organisms that break down the organic matter need nitrogen for rapid and thorough decomposition of the plant material. Therefore, you may wish to occasionally add cottonseed meal or dried blood to provide extra nitrogen.

The composting organisms also need water. To hasten the composting process, keep the pile moist but not soggy. Inadequate moisture will reduce microbial activity, while excess moisture may cause undesirable anaerobic decomposition and unpleasant odors. The pile can also be moistened occasionally with a garden hose during dry periods.

A compost pile will decompose more efficiently if the microorganisms have plenty of oxygen. You can introduce air into the pile by turning it and mixing  it with a spading fork or similar tool.

A pile that has been built in layers, as described above, should be turned for the first time about four weeks after its construction. With a fork, spade, or similar tool, lifting the pile’s lower layers on top of the upper layers to mix the contents. 

Locating two or three compost bins side by side can make the turning process easier and simpler; just shift the material from one bin to the other.

Locating two or three compost bins side by side can make the turning process easier and simpler; just shift the material from one bin to the other. During the warm months, the pile should be turned about once a month. In cooler weather, decomposition is slower and during the winter very little decomposition occurs. A well-managed compost pile will break down in four to six months.

Problems will develop if the pile is not working properly. These problems include unpleasant odor, slow decomposition, a dry pile, or standing water. The solution to a problem depends on its cause. In general, turning the pile, adding water, maintaining a mixture of dry and moist materials, or adding nitrogen will solve the problem.

As the plant materials decompose, the pH of the pile is lowered, meaning that its acidity is increased. However, as the process continues, the pH eventually rises to the neutral level of 7.0 pH. 


Using compost

Compost is ready to use when it is dark and crumbly and the ingredients have lost much of their original identity. Finished compost should have an earthy, pleasant smell.

Once it is ready, the compost can be used as surface mulch on plant beds. Apply compost 2-4 inches deep around vegetable and perennial plants to control weeds and conserve moisture.

Compost also can be mixed into the soil just before planting to lighten heavy soil and improve drainage. It can increase the moisture-holding ability of sandy or light soil.

Adding compost increases earthworm and soil microbial activity that benefits plant growth. Compost can also be used as a substitute for peat moss to amend soil in potting mixes or seed flats, or when transplanting trees and shrubs.