The classic compost pile is very simple: layers or a mix of green & brown plant materials, kept moist and turned.
Compost bins may be constructed from a wide variety of materials -- basically anything that will keep the composting materials confined. Popular materials include wire fencing (either an unsupported closed ring or a U supported with stakes), straw bales, bricks (loose-stacked or mortared), shipping pallets, stacks of tires, or recycled garbage cans. The classic multi-bin composter features a row of three bins constructed of boards; Bin 1 is for fresh material, Bin 2 is "cooking" compost, and Bin 3 is finished compost ready for use.
With increasing interest in backyard composting there are many models of commercial bins available. Often made of durable recycled plastic, commercial bins come in a variety of shapes including cylinders, cones, and pyramids, as well as versions of the traditional square bins.
Sheet composting is just as it sounds -- the materials to be composted are spread out over the surface of the soil.
On-ground composting quickly dries out, and in New Mexico is usually impractical unless done under a layer of mulch. When done in this way -- compostables spread over wet soil, covered with a layer of mulch -- sheet composting is a good way to condition a larger area of ground that can be left unused while the materials work.
Sheet composting is a "cold" process and will not heat up enough to condition the compost. Since it works more slowly and represents a food source, sheet compost may attract a variety of creatures to live under the mulch.
Drum or tumbler composters are designed to make composting as easy and tidy as possible. The classic design is a drum set on casters upon a frame; material is added through a hatch, and the drum is periodically rotated to mix the materials. The mixing distributes oxygen and moisture throughout the compostables, and tumbler composters typically work a batch more quickly than other methods.
Tumblers may be home-crafted from 55-gallon drums or there are many different models on the market, of various sizes and shapes including spherical.
While more work initially than a pile, pit or trench composting is very simple: dig a hole 1-2 feet deep, drop your compostables into it, and cover each layer with some of the excavated dirt.
Trench composting works well through the New Mexico winter, when most gardeners usually have less material to compost and it is more difficult to get a pile to "cook". It is also useful for conditioning difficult patches of soil -- the deeper hole encourages drainage past clay or caliche, and the slow-working materials enrich the soil. Some gardeners plan ahead, placing a compost pit where they will later put a large plant or tree.
Composting in-ground is typically a "cold" process -- the compost works slowly and does not heat up enough to kill weed seeds or insect eggs.
Most composting may involve earthworms at some point. The "cool" composting methods allow earthworms to feed directly on materials; after "hot" piles are worked, earthworms colonize the pile from below.
Vermicomposting involves using a bin specifically designed to allow earthworms access to the composting materials. The bins are typically arranged in a stack; the earthworms move upward through the stacked bins in pursuit of the fresh material added to the top and leave their castings behind.
Bins for worm composting may be made from salvaged materials, or there are numerous commercial products on the market.
Vermicomposting necessarily involves an awareness of the conditions in which the earthworms thrive and an involvement with the process. Children especially enjoy "feeding the earthworms" and seeing garbage transformed into fertilizer. A well-maintained vermicompost bin may even be used indoors.
Properly managed, an earthworm colony can process your kitchen garbage for years to come.
Compost in piles and bins naturally heats up -- this is a side-effect of the microorganisms in the pile breaking down the material. This "cooking" (as gardeners often refer to it) has the desirable effect of destroying many insect eggs, some weed seeds, and many plant diseases. (Of course, it is generally good practice to avoid putting diseased material into any compost pile.)
"Cool" compost, such as pit or sheet composting, does not cook -- the processing of the materials is largely due to earthworms and other soil denizens feasting on the compostables.
Cool composting generally takes longer than hot composting, but does not require the turning that hot piles do.
Aerobic (err-oh-bik) refers to the presence of oxygen. The microorganisms that heat up a compost pile require oxygen; "hot" composting requires that the materials be mixed periodically to allow oxygen to penetrate all parts of the pile.
Microorganisms that do anaerobic (an-err-oh-bik) breakdown typically work more slowly and produce methane as a result.
Both types of organisms live naturally in the soil and have their place in compost, though gardeners usually try to encourage aerobic conditions for their soil and plants.