Issue: March 20, 2004

Compost didn't compost

Last fall I cleaned my garden and made a compost pile of the dead plants and leaves from the trees in the yard. When I went to collect compost from the compost pile, there were still a lot of old leaves and plant stems but not much compost. Why didnât the compost pile make compost?

To form compost quickly, several things are needed. These are a proper balance of carbon and nitrogen in the material being composted, oxygen, and water. Even when things are not optimal, composting occurs slowly.

In New Mexico, water is often a limiting factor. Rain is often not sufficient to keep the composting material sufficiently moist to allow rapid composting. Even in the winter, water must often be added. The proper moisture level is that which keeps the material in the compost pile feeling like a wet sponge after the water has been squeezed out (damp but not wet).

Oxygen is needed because the fungi and bacteria that convert dead plant material into useful compost need the oxygen to respire as they "eat" the garden wastes. In respiration, some of the materials in the compost are converted into sugar, then into carbon dioxide and water by the plant. This process requires oxygen. Energy is also produced in this process, resulting in growth and multiplication of the bacteria and fungi. Some of this energy is the heat that allows composting to continue through the winter and kills weed seeds and disease organisms. Frequent turning (mixing) of the composting material helps mix air into the compost and speeds composting. If you don't turn the compost pile, it will slowly make compost. A proper mix of coarse and fine particles also helps with aeration. If the composting materials are too fine, the pile becomes anaerobic and foul smells develop. The composting process may slow under these conditions. If the particles are too coarse, then there is too much air exchange, and drying and loss of heat slow the composting process.

Finally, a proper balance of carbon containing materials and nitrogen containing materials is necessary. Dead, dry (often brown) plant material is usually high in carbon. The carbon is necessary, but it must be present in the proper ratio with the nitrogen (green plant material, manure, and nitrogen fertilizer). This ratio should be about 20 to 30 parts carbon for 1 part nitrogen. If the balance is too high in nitrogen, ammonia smells develop and useful nitrogen is lost into the atmosphere. If carbon is too high, composting proceeds at a slow rate.

For more information you can find publications on composting at your local Cooperative Extension Service office or online at http://www.cahe.nmsu.edu/pubs/_h. Dr. George Dickerson, NMSU Extension Horticulture Specialist has written the following publications that will be helpful: H-110: Backyard Composting and H-164: Vermicomposting (composting with worms).

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!