Probably the best homemade composting design is the multiple bin
system where separate compartments facilitate continuous
decomposition. Each bin is about four feet on a side and three to
four feet tall. Usually, the dividing walls between bins are shared.
Always, each bin opens completely at the front. I think the best
design has removable slatted separators between a series of four
(not three) wooden bins in three declining sizes: two large, one
medium-large and one smaller. Alternatively, bins may be constructed
of unmortared concrete blocks with removable wooden fronts.
Permanently constructed bins of mortared concrete block or wood may
have moisture-retentive, rain-protective hinged lids.
There are two workable composting systems that fit these structures.
Most composters obtain materials too gradually to make a large heap
all at once. In this case my suggestion is the four-bin system,
using one large bin as a storage area for dry vegetation. Begin
composting in bin two by mixing the dry contents temporarily stored
in bin one with kitchen garbage, grass clippings and etc. Once bin
two is filled and heating, remove its front slats and the side slats
separating it from bin three and turn the pile into bin three,
gradually reinserting side slats as bin three is filled. Bin three,
being about two-thirds the size of bin two, will be filled to the
brim. A new pile can be forming in bin two while bin three is
cooking.
When bin three has settled significantly, repeat the process,
turning bin three into bin four, etc. By the time the material has
reheated in bin four and cooled you will have finished or
close-to-finished compost At any point during this turning that
resistant, unrotted material is discovered, instead of passing it
on, it may be thrown back to an earlier bin to go through yet
another decomposition stage. Perhaps the cleverest design of this
type takes advantage of any significant slope or hill available to a
lazy gardener and places a series of separate bins one above the
next, eliminating any need for removable side-slats while making
tossing compost down to the next container relatively easy.
A simply constructed alternative avoids making removable slats
between bins or of lifting the material over the walls to toss it
from bin to bin. Here, each bin is treated as a separate and
discrete compost process. When it is time to turn the heap, the
front is removed and the heap is turned right back into its original
container. To accomplish this it may be necessary to first shovel
about half of the material out of the bin onto a work area, then
turn what is remaining in the bin and then cover it with what was
shoveled out. Gradually the material in the bin shrinks and
decomposes. When finished, the compost will fill only a small
fraction of the bin's volume.
My clever students at the Urban Farm Class, University of Oregon
have made a very inexpensive compost bin structure of this type
using recycled industrial wood pallets. They are held erect by
nailing them to pressure-treated fence posts sunk into the earth.
The removable doors are also pallets, hooked on with bailing wire.
The flimsy pallets rot in a couple of years but obtaining more free
pallets is easy. If I were building a more finished three or four
bin series, I would use rot-resistant wood like cedar and/or
thoroughly paint the wood with a non-phytotoxic wood preservative
like Cuprinol (copper napthanate). Cuprinol is not as permanent as
other types of wood preservatives and may have to be reapplied every
two or three years.
Bins reduce moisture loss and wood bins have the additional
advantage of being fairly good thermal insulators: one inch of wood
is as much insulation as one foot of solid concrete. Composting
containers also have a potential disadvantage-reducing air flow,
slowing decomposition, and possibly making the process go anaerobic.
Should this happen air flow can be improved by supporting the heap
on a slatted floor made of up-ended Cuprinol-treated 2 x 4's about
three inches apart tacked into the back wall. Air ducts,
inexpensively made from perforated plastic septic system leach line,
are laid between the slats to greatly enhance air flow. I wouldn't
initially build a bin array with ducted floors; these can be added
as an afterthought if necessary.
Much simpler bins can be constructed out of 2" x 4" mesh x 36" or
48" high strong, welded wire fencing commonly called "turkey wire,"
or "hog wire." The fencing is formed into cylinders four to five
feet in diameter. I think a serious gardener might need one
five-foot circle and two, four-foot diameter ones. Turkey wire is
stiff enough to support itself when formed into a circle by hooking
the fencing upon itself. This home-rolled wire bin system is the
least expensive of all.
As compostable materials are available, the wire circle is gradually
filled. Once the bin has been loaded and has settled somewhat, the
wire may be unhooked and peeled away; the material will hold itself
in a cylindrical shape without further support. After a month or two
the heap will have settled significantly and will be ready to be
turned into a smaller wire cylinder. Again, the material is allowed
to settle and then, if desired, the wire may be removed to be used
again to form another neatly-shaped heap.
Wire-enclosed heaps encourage air circulation, but can also
encourage drying out. Their proper location is in full shade. In
hot, dry climates, moisture retention can be improved by wrapping a
length of plastic sheeting around the outside of the circle and if
necessary, by draping another plastic sheet over the top. However,
doing this limits air flow and prevents removal of the wire support
You may have to experiment with how much moisture-retention the heap
can stand without going anaerobic. To calculate the length of wire
(circumference) necessary to enclose any desired diameter, use the
formula Circumference = Diameter x 3.14. For example, to make a
five-foot circle: 5 x 3.14 = approximately 16 feet of wire.
With the exception of the "tumbler," commercially made compost bins
are derived from one of these two systems. Usually the factory-made
wire bins are formed into rectangles instead of circles and may be
made of PVC coated steel instead of galvanized wire. I see no
advantage in buying a wire bin over making one, other than
supporting unnecessary stages of manufacture and distribution by
spending more money. Turkey wire fencing is relatively inexpensive
and easy enough to find at farm supply and fencing stores. The last
time I purchased any it was sold by the lineal foot much as hardware
cloth is dispensed at hardware and building supply stores.
Manufactured solid-sided bins are usually constructed of sheet steel
or recycled plastic. In cool climates there is an advantage to
tightly constructed plastic walls that retain heat and facilitate
decomposition of smaller thermal masses. Precise construction also
prevents access by larger vermin and pets. Mice, on the other hand,
are capable of squeezing through amazingly small openings.
Promotional materials make composting in pre-manufactured bins seem
easy, self-righteously ecological, and effortless. However, there
are drawbacks.
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.
There are other less obvious tumbler disadvantages that may negate
any work avoided, time saved, or sweaty turning with a manure fork
eliminated. Being top-loaded means lifting compost materials and
dropping them into a small opening that may be shoulder height or
more. These materials may include a sloppy bucket of kitchen
garbage. Then, a tumbler _must_ be tumbled for a few minutes every
two or three days. Cranking the lever or grunting with the barrel
may seem like fun at first but it can get old fast. Decomposition in
an untumbled tumbler slows down to a crawl.
Both the passive compost bin and the highly active compost tumbler
work much better when loaded with small-sized particles. The
purchase of either one tends to impel the gardener to also buy
something to cut and/or grind compost materials.