So, what can you use to make compost and what should you avoid?
A variety of materials can be composted, but most Gardeners want to recycle collected yard waste. Plants lose between 50 and 75 percent of their volume in composting, so a lot of plant material can be processed effectively.
Composting can be effective on most yard wastes such as leaves, vegetable and flower plant parts, straw, and a limited amount of woody prunings, grass clippings and weeds. Woody twigs and branches that are greater than 1/4 inch in diameter should first be put through a shredder-chipper. Avoid highly resinous wood and leaf prunings from plants such as junipers, pine, spruce and arborvitae. The resins protect these materials from decomposition and extend the time needed for composting in comparison with other plant materials. High tannin-containing leaves (oak and cottonwood) have similar problems but can be used in small quantities if chopped well and mixed with other materials. The easiest way to handle grass clippings is to leave them on the lawn. Research shows that they return valuable nutrients back to the soil. Some grass clippings can be used for compost if other green plant material isn't available.
Many, but not all, plant disease organisms are killed if the compost reaches 122 degrees F. Temperatures will vary throughout the compost. Outer layers stay cooler than the center and cause uneven kill of disease organisms. If a plant is severely diseased, it is better to dispose of it in the trash.
In general, avoid plants treated with weed killers. Small amounts of herbicide-treated plants may be mixed in the pile as long as you allow for thorough decomposition. Weed killers and other pesticides break down at various rates. If you use treated grass clippings, the breakdown of these chemicals should be at least as fast as breakdown in the soil. Plants killed with weed killers that are soil inactive (glyphosate products such as Roundup or Kleenup) should present no problem when composted in small quantities.
In addition to yard wastes, many people compost kitchen wastes, such as vegetable scraps, coffee grounds and eggshells. These materials compost well and usually are not produced in large enough quantities to displace yard wastes. Animal wastes (meat, bones, grease, whole eggs and dairy products) may cause odors and attract rodents; they are not recommended. Human, cat or dog feces may transmit diseases and should not be used. Some animal products that can be used as organic sources of nitrogen include blood and steamed bone meal and livestock manures from plant-eating animals (cows, sheep, rabbits and chickens).
Manures may contain new strains of E. coli and other bacteria that cause human illness. The use of manures added directly to the food Garden is questionable, although use on ornamental plantings is still recommended. Research shows that 2 to 10 percent of bacterial pathogens survive the composting process. If manure is composted for food Gardens, a two- to four-month curing process following composting is necessary to reduce pathogens. Favorable moisture and temperature conditions during curing allow microorganisms to develop and outcompete the pathogens. Thoroughly wash all fruits and vegetables and peel according to safe food handling practices to minimize the possibility of bacteria-contaminated soil being carried into food prepared for human consumption.
Black and white newsprint is best recycled through recycling collection operations rather than converted to compost. If paper is composted due to a shortage of dry materials, add no more than 10 percent of the total weight of the material being composted. Do not use wood ashes or lime for composting in Colorado. Both increase salt and alkalinity, which leads to a loss of nitrogen in the form of ammonia gas.
Catching and bagging grass clippings is not worth the effort when they are easily recycled right on the lawn. For details, see fact sheet 7.007, Eliminate Grass Clipping Collection. If clippings are too long to leave on a lawn, composting is a better alternative than disposal in the trash. Some weeds can be composted, particularly if they are pulled before they produce seed. Compost mainly serves to reduce the volume of yard wastes and convert plant materials into a usable soil amendment. Adding excessive amounts of other materials, such as animal manures, defeats the purpose.