One reason for a deeper VF would be (assuming a bog design with an actual
soil bed), to allow anaerobic conditions to occur in the deeper soil layers,
which allows for the reduction of nitrates back to nitrogen, which is more
efficiently utilized by all the plants.
Plants can use nitrates, but they require more energy to do so, and the
process is far less efficient than using (reduced) nitrogen.
Use of deep soil, in the bog approach, is referred to as a "nitrate sump".
You can watch the nitrate levels in the pond go down resulting in less algae
issues.
If you are strictly doing a higher water flow VF, without any significant
depth of soil, you really would only need you VF deep enough to accommodate
the root system. (ie 6 inches). Too deep would impede the flow of oxygenated
water, and you could get anaerobic conditions in the deeper layers of the
water as well, though I doubt nitrate reduction would occur anywhere near as
well as in the bog model with deeper soil.
I went with the bog model, which has worked well, but my soil depth is only
about 12 inches. Nitrates went down, after the 3rd year when the plants were
well established.
Happy ponding,
Greg


"John Rutz" wrote in message
...


Paul in Redland wrote:
One ft should be fine. Are you putting your plants in pots?
Paul




mine varies from 1 ft to 18 in due to root size of some plants

--
John Rutz

I'm not Porg am not am not am so
see my pond at:
http://www.fuerjefe.com