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.