Many backyard gardeners are familiar with composting, a process which recycles yard trimmings along with vegetable and fruit scraps from the kitchen.  Compost, a humus-like soil, results from the biological breakdown that occurs from microbial decomposition of organic material.  Composting enriches soil, reduces waste in landfills, and conserves water. 

The compost process depends upon four key ingredients: carbon, nitrogen, oxygen, and water.  The mixture of materials in healthy compost should have a carbon to nitrogen ratio of three to one.  This can be accomplished by including equal amounts of “browns” (carbon-rich materials) and “greens” (nitrogen-rich materials).  Examples of “browns” include dry leaves, straw, sawdust, and wood chips.  Paper, cardboard, and newspaper may also be used as “browns,” but should first be shredded or chopped.  Examples of “greens” include vegetable and fruit scraps, coffee grounds, grass clippings, manures, and wilted flowers.  All materials decompose best if they are less than two inches in size.  Many gardeners use a lawn mower for shredding dry leaves, and a shovel or clippers for chopping woody materials. 

To create compost, oxygen and water should also be balanced (50 percent moisture plus 50 percent oxygen).  It is important to consider the moisture content of added material such as food scraps and freshly cut grass prior to adding water.  Compost should be about as moist as a wrung-out sponge, moist to the touch but not yielding liquid when squeezed.  Turning the compost pile aerates it by incorporating oxygen while mixing the materials.  Some compost bins available commercially are designed to turn, eliminating the need to building the pile in layers (and also eliminating the need for a hay fork).  Bear in mind that high summer temperatures will dry the pile more rapidly. 

Temperature is another critical factor in composting.  It is affected by the size of the pile and by the oxygen and moisture content.  Heat is generated by the microorganisms as they break down the organic materials.   Temperatures of about 160 degrees provide the best environment for the microorganisms to do their work.   Heat retention is best achieved in covered bins rather than open piles. Turning the pile will help to prevent overheating.  If the microorganisms are killed by becoming too hot, the pile will cool and the whole composting process will have to be started again.  This is why many gardeners swear by a three-bin method, transferring and turning over materials from one bin to the next every few weeks.  Specialized thermometers are useful in maintaining the proper temperature.

The length of time for composting depends on several factors: the density and size of the materials, the carbon and nitrogen content, moisture content, aeration, and volume.  If you continue to add materials to your compost pile or bin the process will take longer.  Turning the pile or bin on a regular basis is the key to having compost ready for your gardening needs. Various sources state that composting can take as little as two weeks and as long as two years.   Finished compost is a dark brown, easily crumbled material with a musty smell.  The original volume of the compost material is significantly reduced, as is the temperature.  The compost now is ready to be used.  It can be screened through wire mesh to sift out any larger pieces that have not decomposed. 

Once the compost is ready it may be used as mulch around trees, shrubs and other plants. Compost will help suppress weeds while at the same time increasing moisture retention.  It may also be incorporated into the soil to improve soil quality.  The nutrients in compost are slowly released into the soil and are more easily available to your plants.  Once you have started using your own compost you will be convinced that this method of recycling is beneficial on many levels.   

For further information, see Pamela M. Geisel and Donna C. Seaver's UC Publication 8367, Composting Is Good for Your Garden and the Environment (PDF).

To learn more about different methods and techniques for making compost, join our Master Gardener workshop on Composting, October 27.  For descriptions of this and all the other workshops in the Master Gardeners' Fall Workshop Series, visit our website.  All workshops are free, but registration is required.

UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system.  To learn more about us and our upcoming events, and for help with gardening in our area visit our website.  If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu or leave a phone message on our Hotline at 530-552-5812. To speak to a Master Gardener about a gardening issue, or to drop by the MG office during Hotline hours, see the most current information on our Ask Us section of our website.