It is not possible to readily turn the materials once they’ve been placed into most composters of this type unless the entire front is removable.  Instead, new materials are continuously placed on top while an opening at the bottom permits the gardener to scrape out finished compost in small quantities.  Because no turning is involved, this method is called “passive” composting.  But to work well, the ingredients must not be too coarse and must be well mixed before loading.

Continuous bin composters generally work fast enough when processing_ mixtures of readily decomposable materials like kitchen garbage, weeds, grass clippings and some leaves.  But if the load contains too much fine grass or other gooey stuff and goes anaerobic, a special compost aerator must be used to loosen it up.

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.

Tumblers have disadvantages that may not become apparent until a person has used one for awhile.  First, although greatly accelerated, composting in them is not instantaneous.  Passive bins are continuous processors while (with the exception of one unique design) tumblers are “batch” processors, meaning that they are first loaded and then the entire load is decomposed to finished compost.  What does a person do with newly acquired kitchen garbage and other waste during the two to six weeks that they are tumbling a batch?  One handy solution is to buy two tumblers and be filling one while the other is working, but tumblers aren’t cheap!  The more substantial ones cost $250 to $400 plus freight.