Compost

Why compost?

New Cauliflower

New Cauliflower

Compost is the product of aerobic decomposition of organic materials in the process of composting. The most common purpose for compost is in agriculture and horticulture to condition soils and provide nutrients for plants. Once mixed into the soil, the compost becomes humus. Compared with other forms of soil amendment, composting is highly efficient because the process recycles materials that are already on site rather than having to haul them away for disposal. The best part of composting is that microbes in the soil do most of work for free.

In sandy soil, compost soaks up water and nutrients and releases them slowly to plant roots. In clay soil, compost forms a base for clay particles to accumulate into granules making the soil more permeable for water, air, roots, and microorganisms that break down organic matter and minerals.

How to Compost

The organic materials to be composted are placed in layers forming communities for the microorganisms that break down each particular community. Keeping the pile evenly moist provides enough water for the microorganisms without drowning them. When a handful of compost is squeezed, it should be about as damp as a well wrung sponge.

The microorganisms use nitrogen in the process of decomposition so the addition of a small amount of nitrogen can help to break down the organic matter faster. The optimal amount of nitrogen in the pile is based on the amount of carbon. Plants are mostly carbon and water. Some possible sources of nitrogen are: high nitrogen fertilizer, the first grass clippings of spring, leaves, and compost starters or activators which might contain nitrogen, bacterial, or fungal microorganism to quick start the decomposition.

Microorganisms create heat while metabolizing organic matter and an appropriate amount of heat increases the activity of microorganisms. The optimal temperature for a compost pile is around 130°F but no more than 140°F. In Tucson, composting may benefit from sun or insulation during the winter to keep the temperature warm and from shade or insulation in the summer to keep the temperature from getting too warm which wouldinhibit or kill the beneficial bacteria and fungi. A thermometer can be used to determine the general health of the compost pile. After the compost is broken down, it can be removed from the pile and heated to as high as 180°F for a half hour or so to kill pathogens and weed seeds.

In aerobic decomposition, microorganisms use oxygen from the air and expel carbon dioxide as a waste product. For healthy decomposition, the pile should be turned occasionally to replace the carbon dioxide with oxygen contained in fresh air. Special tools are produced for this function but hoes and shovels will also work.

commercially produced composter
Composter from Tucson Recycles program.

A variety of different types of containers can be used for composting including commercially produced compost bins. The City of Tucson has held (and might again hold) one-day subsidized compost bin sales. Watch for announcements on the TucsonRecycles Website. The container should be sturdy and allow the materials being composted to be in direct contact with the underlying soil to allow bacteria, fungi, and slightly larger critters in the soil to get into the compost. In some areas were furry desert varmints could become a problem, 1/4 inch square mesh galvanized hardware cloth spread under the composter will be permiable to the microorganisms while preventing the larger wildlife from building a home in the warm compost.

DIY compost containers can be made in several ways: cut the bottom out of an old waste container; make a circular fence using chicken wire and line it with cardboard then cover it with an old rug to retain moisture; a bottomless wood box made of boards or plywood can work well, make a multi-compartment bin by stacking concrete blocks for sides. When using hollow concrete blocks, placing concrete pavers above and below the blocks will keep rodents from nesting in the openings.

Some commercially available compost bins are designed to be easily rotated end-over-end as a labor saving method to keep the compost aerated.

Chopping the organic material into small pieces before adding to the compost will help decomposition by providing more surface area for the microorganisms and nutrients. Turning over the compost every week or two will keep more surface area exposed to fresh oxygen. Selecting a container with a few small holes near the bottom for ventilation might help, but there should not be so much ventilation that it dries out the sides of the compost.

What Causes Odors

Anaerobic decomposition--decomposition without oxygen--can give the compost pile a rotten smell. The compost must be turned over enough to ensure a sufficient supply of fresh oxygen and must not be watered excessively which would also deprive the compost of oxygen. When squeezing a handful of compost, it should have the moisture content of a well wrung damp sponge.

Excessive nitrogen can turn into ammonia. Hold back on adding nitrogen or add more organic matterials.

What to Compost

Wood such as tree chips or sawdust are particularly good for clay soil but nitrogen should be added to make up for the large amount of nitrogen used up in the decomposition of wood. 1/2 lb. of nitrogen should be added to every 10 cubic feet of wood compost. A tad more than 2 lbs. of ammonium sulfate, about 4-1/2 lbs. of 11–52–0 mono-ammonium phosphate, or about 3 lbs. of 16–20–0 di-ammonium phosphate will provide 1/2 lb. of nitrogen.

What NOT to Compost

What to Do With Compost

A rectangular frame made from scrap 2x4's and covered on one side with 1/4 inch mesh hardware cloth will make a handy screen to separate the smaller bits of finished compost from larger pieces that should go back into the composter. Make the frame a size that can sit on top of a wheelbarrow. The same sifter can also be used to separate rocks from soil.

Spread the sifted compost onto the surface of your garden and work it into the soil. Lawns will benefit by working in some compost during the spring or fall. When adding compost around trees and shrubs, take care to keep the compost from piling up against the trunks which could cause disease.

Worms in Tucson?

Vermicomposting ...The problem in Tucson seems to be how to keep the worm farm temperature from rising above about 70°F during the summer when the outside temperature can reach above 110°F. This will take some thought.

Composting Prickly Pear?

Cattle ranchers are composting Prickly Pear cactus which proves that it can be done but I have yet to experiment with this although I plan to try it soon. If anybody has experience composting Prickly Pear cactus for home gardens, please let me know.