Organic Gardener's Composting by Steve Solomon

Manufactured passive composters are not very large. Compactness may be an advantage to people with very small yards or who may want to compost on their terrace or porch. But if the C/N of the materials is not favorable, decomposition can take a long, long time and several bins may have to be used in tandem. Unless they are first ground or chopped very finely, larger more resistant materials like corn, Brussels sprouts, sunflower stalks, cabbage stumps, shrub prunings, etc. will "constipate" a top-loading, bottom-discharging composter.

The compost tumbler is a clever method that accelerates decomposition by improving aeration and facilitating frequent turning. A rotating drum holding from eight to eighteen bushels (the larger sizes look like a squat, fat, oversized oil drum) is suspended above the ground, top-loaded with organic matter, and then tumbled every few days for a few weeks until the materials have decomposed. Then the door is opened and finished compost falls out the bottom.

Tumblers have real advantages. Frequent turning greatly increases air supply and accelerates the process. Most tumblers retard moisture loss too because they are made of solid material, either heavy plastic or steel with small air vents. Being suspended above ground makes them immune to vermin and frequent turning makes it impossible for flies to breed.