Since everyone should have a pretty good idea where we're at with this thing, I think we will just dispense with the formalities, and jump right in. We were discussing the size of worm-bins, and the time has come to explain why I feel the commonly-accepted sizes are in need of some rethinking. It should be kept in mind 
that our original objective involved maintaining a large population of red worms (for future sale), in as small an area, and with as little trouble, as possible.
In virtually every book written on the subject of vermicomposting, the matter of "bedding depth" is generally agreed upon. The suggestions may vary by a couple inches from one volume to the next, but almost always fall into an accepted range of somewhere between 6-12 inches (and most often 8-10 inches.) These recommended depths are then supported by the "fact" that red worms dwell most commonly in the top 4-6 inches of their environment (a statement I accept as true.) The very nature of worm "culturing", however, is to establish conditions which are most beneficial for the worm (if it is to thrive), and for the "breeder" (who would like to prosper.) This may, at times, involve adjusting some of the conditions which we regard as normal (worms are not naturally found living in houses), and I believe that bedding depth is one of the "natural" conditions which can, and should be manipulated (for the benefit of both the worm, and the breeder.) First, however, let's take a look at why red worms are "normally" found so close to the surface.
One point I had decided on in my earlier studies of this subject (not necessarily correctly), was that red worms required a larger amount of nutrient intake than their soil-dwelling cousins, in order to survive in a healthy condition (the amount of nutrients available in compost or manure would put virtually any soil to shame.) Since material dropped on the ground very seldom "sinks" to any depth, the upper regions of the soil became the red worm's natural habitat. According to that train of thought, if I located the food source lower in the soil, the red worms would move down in search of it, take up residence in the proper area, and live happily ever after. When this appeared to be actually happening, I felt rather pleased with myself. When the worms started shrinking (and after I determined they were still healthy...just very tiny), I assumed it had to do strictly with the inherent dfficulties a "litter-dwelling" worm encountered when it tried to adapt to living in dense soil. Though I still believe that to be partially correct (consider our earlier "bucket" experiment), I now realize there were even more accidental discoveries involved in "shrinking" my worms than I had realized at first. (I have never denied this whole procedure came about as a series of "accidents.")
After a fair amount of thought, testing, and successful reroduction of the results, these are my best ideas on what happened. It was a long time ago that I stated, there are still aspects of this procedure that I am trying to figure out, but whether all the particulars are here or not, I do know if you duplicate these conditions, you will witness similar events. After I give you my best explanation of the situation, I will point out the reasons you might want to consider setting up your system in a similar manner.
If you remember way back at the beginning, I told you the size of the bin-system I described was important to duplicating these results. This is why. Having a 2-foot by 3-foot, by 30-inch area to work with (one third of the master-bed), allowed me to try something you simply cannot do in a smaller situation (if you love your worms, don't even think about trying this in a small bed.
) Hollowing out a full third of the first compartment, to a depth reaching within 2 inches of the bottom, I proceeded to fill the opening with raw compost materials (food scraps, grass clippings, leaves, commercial feed, and even a used vacuum-cleaner bag.) I covered this trench with 3-4 inches of bedding, which meant I had stuffed roughly 25-30 pounds of organic material into a single bed, all at one time. Guess what?
A day later I was able to get a temperature reading from that trench of nearly 130 degrees farenheit (now you know why I said not to try this in any bed that is not large enough to give the worms plenty of space to retreat from the heat....nice rhyme.) The heating only lasted a day or two, however, and a few more days after that, the bedding level on the side of the bin with the trench, started dropping fairly rapidly. About 14 days after the first trench was installed, I repeated the procedure on the opposite side of the bin (I never used the center area, as a safety measure in case both sides ended up heating at the same time, and also to make sure that the fresher material, didn't start whatever was left of the old material, heating once again.)
By the time a couple months had passed (and I had all three beds working in the same fashion), a few noteworthy things had occurred.
At that particular point, the worms were dwelling throughout the bedding, top to bottom, and were still maintaining their "normal" size. Feeling rather burnt out from the sustained effort of keeping these critters in munchies, I now humbly admit to a certain period of "neglect" where these worms were concerned. Where each of the individual beds had been receiving between 50-125 pounds of feed a month, Willy's friends were now lucky if I remembered to water them occasionally (I even told myself I was merely letting them clean everything up in preparation for new bedding.) I guess my next discovery was what this article is all about.
It came about that I got asked to set up a new bin for a gentleman, and of course, supply him with the starter batch of worms. I whipped together a three-bin system (much smaller than my own), and when the time arrived to install the worms, I went down to "harvest" them. The first thing I noticed was there appeared to be no adults in any of the areas I tried to harvest....lots of spawn, but not a single adult (sound familiar?) Try as I might, I simply couldn't locate the sexually-mature worms responsible for all those spawn. When I realized I was facing the same situation in all three beds, I assumed the adults had all died due to lack of food, and decided on the only alternative I had. I harvested an entire pound of these ridiculously small red worms, and installed them into the gentleman's bin-system. Well, there is no need to bore you with the details, since you already know what had happened. It turns out, if that customer had had sufficient room to grow those worms he received to their full size, he would have had hundreds of pounds of worms for the cost of one.
At any rate, it appears that while those beds were being supplied with the enormous amounts of feed material which I began with, the resident worms were adding a whole new meaning to the word "orgy." Then, when the food supply dried up (and I thought they had died off), they shrunk in response to the new situation. With normal breeding (while in the shrunken state), they filled the beds to capacity over the next few months, and thus, even when I caught my interest again, and tried returning to the original feeding schedual, they were unable to grow, since the beds just couldn't accomodate that many "full-sized" worms. By simply removing a number of these worms to a "private" bed, with lots of room and food, I was able to grow them back to "normal" size in what was very often, only a matter of 10-15 days. That's how my system of raising lots of worms in a small area came to be, and why I can't tell you so easily how exactly it worked (it's a combination of several different circumstances.) What's really important, however, is what this means to the person who wishes to maintain a large stock (in number) of worms, in a small area, without having to feed them hundreds, or thousands of pounds of feed. And this (finally), brings us back to the matter of depth.(Was that a digression? I think that was a digression....Sorry.)
Consider first of all, the usual recommended depth of a worm bin....10 inches for the sake of argument. We'll start with adult (full-sized) worms just to keep matters simple. If your worm-bin measures 2 feet by 3 feet, and is 10 inches deep, you have space (according to most of the manuals) for roughly 6000 worms, or to put it another way, 6 pounds of adult red worms (24,000 bed-run.) If you increase your depth to 20 inches (costing you next to nothing in floor-space), you double the available bedding area, allowing twice as many worms. This may not seem like a lot, but remember that we are considering "shrinking" our worms, which means that a 10-inch depth will accomodate (and this figure is extremely modest) 120,000 adult worms....and 20 inches allows for almost a quarter million. These worms will still only weigh 12 pounds, and have only to be fed accordingly. That means you can feed the "equivalent" of 250 pounds of "full-sized" worms, on the diet normally required by only 12 pounds of worms! This is a major difference, in both effort, and in maintenance costs. And there are still other benefits to the deeper bed.
The most significant benefit of the deeper bed lies in the fact that you can feed much larger quantities of food at once (remember to be careful about heating.) This results in a lot less work for you, a wider variety of suitable feed materials (and less need of grinding scraps to conserve space), as well as reducing the frequency with which the worms are disturbed (something I know from experience, leads to improved reproduction rates, as well as healthier worms.) In addition to all this, the added depth provides a wider selection of "micro-climates" for the worms to choose from. Thus, if the bed dries out a little too much near the surface, the worms can move lower down, and vice versa if the lower regions get a little too wet. (In regards to this, one researcher set up a worm bin with multiple levels, designed like drawers. He was able to determine what the moisture content was at each level, and the fact that 85% of the worms congregated at one specific depth showed that they have definite preferences, and will seek out suitable conditions. They have little, or no opportunity to do so in a bed that has no significant depth. And remember... happy worms will always mean healthier worms.
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At any rate, due to the length of this entire 7-part article, and my own inability to avoid digressing, I am reasonably sure I have forgotten things I meant to get around to, and have probably produced as many questions for some of you, as answers. For that, you have my apologies, and assurance that I will try to answer any questions you care to send my way, regarding this matter. If anything contained anywhere in this piece has helped anyone in any way, then I guess it wasn't a total failure, and I can only promise to try and do better in the future.
Until then, however,....The End.
Some of These Icons obtained from Ender's Realm
Some of These Icons obtained from Leo's Icon Archive
The Burrow