Another idea about this that I've wondered about from time to time, is whether a really simplified version of this would "work" OK.  If you think of the extreme example where the wicking chamber is 100% of the cross sectional area, that's just like a solid-bottomed pot with drain holes somewhere on the sides.  So I wonder if it would work out to just have a solid-bottomed pot, and drill some drain holes some ways up the sides of the pot, and when you load it just insert some kind of water-permeable barrier (e.g. a screen or heavy landscape cloth, or both) at a height just above where the lowest drain hole is.  That'd be like a "root redirector" or act to encourage some root clustering above the barrier.  Then always fill with water until water drains out the side drain holes.  Meanwhile, for aeration, at that same height (just above the barrier) have a couple of horizonal airspace tubes... i.e. perforated pipes poked through horizontally at that elevation, with ends open to allow air circulation.  I guess the barrier would just discourage (but likely not actually prevent) roots from growing down into the "wet feet" area of the combined reservoir/wicking chamber, but I wonder if it'd work.  Maybe the drowning effect of any roots that did grow down there (in the "wet feet" zone), combined with the mild deterrent of the barrier, would be enough to encourage root development higher up (in the non-drowned soil chamber zone)?  Don't know (and never tried it), but I've wondered.  It seems to me it'd be simpler to make.

Mike

I would agree that depth of the soil is not as critical a dimension as wick cross sectional area, and its  partly due to the wicking ability (capillary action) of the soil mixes. If the depth is too deep the upper portions of the soil will not get watered. The maximum depth of successful larger SIPs is usually around 18 inches with an average of 12 - 16 inches. the reservoir can be any depth, but should not be too deep because the water has to be wicked up into the potting mix through capillary action / wicking ability of the potting mix.

On average the reservoirs are 4" - 6" deep, they could be shallower if an automated watering system is used, but the reservoirs also serve the secondary purpose of being a container for the long "water" roots that develop later in the growing season. The reservoirs are also kept relatively shallow so that the water is wicked away / removed / exchanged continuously and doesn't stagnate. A simple automated watering system is the Level Control Bucket as used in the Alaska Grow bucket systems,  http://alaskagrowbuckets.com/alaska-grow-bucket-guide , the level control bucket can be used with any container and containers can be grouped to be watered by a single Level Control Bucket attached to the water supply.

The wick cross section is one of the most import items for a successful SIP and IMO, it should be ~ 10% of the containers cross section (Area)... 12 " diameter container 4" wick,  18" diameter 6" wick, 24" diameter 8" wick etc... These numbers are from experimentation and measurements of the wick area / planter area of Commercially Manufactured SIPs. A larger cross section results in faster saturation, while smaller results in reduced capillary action and water into the potting mix.
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The potting mix also has to have a high soil porosity (wicking ability, high portion of fine particles) which means the high peat content of commercially available mixes, usually more than 70%, for the increased capillary action. 
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An example of the main design principle of SIPs, Saturation Zone / Perched Water aka the soil wick.
<edit> Attached Pics.
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Pete S. / NY,  Zone 6A

Good info Pete, thanks.  I agree with the points you're making, particularly about soil porosity, and the other size descriptions you gave.  The only part I'm not sure about is the wick cross section of ~10%.  I think that this percentage varies depending on (among other things) the soil depth.  Greater soil depth implies a need for greater percentage of wick cross section  (if all other things are held constant).  I used a higher percentage than 10%, more like 20%, using rectangular containers with multiple 4" circular wicking chambers (typically sold as "perf pipe" for drainage applications, but made of drinking-water-safe plastic, like well pipe).  (Tangential note to readers:  I don't recommend that you use PVC for this... PVC is not food safe, and you'll leach lots of nasty stuff into your soil and your fruit... I was particularly disappointed to see a few "organic gardener" sites on the web that recommended using PVC pipe as wicking chambers... they just didn't research it and they gave bad advice).  OK, end of tangent.  So, using about 20% surface area for the wicking chambers, I didn't see any negative results.  What negatives would accrue from faster saturation?  As long as I used soil that was loose enough to stay well aerated, I didn't see any negative consequence.   (maybe it's dependent on the kind of plants you put in the SIPs... i.e. tomatoes loved it, but they tolerate more moisture well).

Good info Pete.

Mike

Mike,
Thanks. The PVC issue is why I've used the Corrugated HDPE (High Density Polyethylene) drain pipe and HDPE water pipe as fill pipe in the fabrication of my SIPs, but there is PVC rated for PW (Potable Water) that is currently available on the market. Its listed right on the pipe.

The Wick cross section of ~10% works with all the higher porosity / high peat content mixes (as listed on the Earth Box website). The Custom mixes have to be tested in the SIPs for proper wicking, and lower porosity potting mixes also fall into this category. After the roots are established the plants are able to acclimate themselves to their growing environment, especially since the SIPs have the added advantage of maintaining better aeration than top watered containers, which may be a reason why the Mushroom Compost works in the Bill's SIPs. Yes, Tomatoes do great in SIPs and usually do not have a problem with "wet feet".

Thanks Pete.  HDPE is the stuff I use too.  4" corrugated perf pipe.  I didn't know there's PVC rated for drinking water (but I still won't trust it... it's probably coated or something, because PVC itself is inherently leachy of the "VC" (vinyl chloride) part).  Anyway, glad it's all working out with your SIPs.  

Mike