Hi all,
Yes a bit of a quick reply so "less haste, more speed".
I am, however, not sure if anyone who has forked out £100 on a complete substrate would agree with you opinions - "If you fertilise the water column the substrate becomes relatively unimportant for many plants as a nutrient source, it is there purely to anchor the plant."
I should have said that some plants, Cryptocorynes for example, will do better with a nutrient rich substrate, and that a complex natural mix of clays and humus may offer some small advantages in terms of growth and nutrient retention & availability over a less complex mix of (calcined) clays and humus (laterite, Akadama and sphagnum peat or "kitty litter" and leaf mould). In both cases assuming that the mix has a reasonable CEC they will perform the same function. I now use a 80% silica sand/10% clay/10% leaf mould based substrate. This is because I am interested in limiting nutrients, if I was going for maximal plant growth I would use a more nutrient and humus rich substrate (probably by adding "Osmocote" or similar controlled release fertiliser, and more clay and humus), as well as adding CO2 and nutrients to the water column (as EI or similar).
Are anaerobic areas common in our tanks, Darryl? I was under the impression that this wasn`t the case, but it is something I am not too sure of.
The answer to this depends upon 3 factors, the nature of the substrate (depth, particle size, physical structure of the particles ), how highly oxygenated the water in contact with the substrate is and the BOD (Biochemical Oxygen Demand) of the whole system. A further factor is the extent and penetration of the roots (and rhizosphere) of the plants.
If you have a thin layer of large spherical inert substrate (think marbles or rounded flint gravel) it will have no CEC and minimal biofilm development and it will remain fully oxygenated unless the water flowing through it becomes de-oxygenated (realistically by the tank inhabitants/chemical oxygen demand/ dead fish etc. before it reaches the substrate). In most(all?) other cases their will be zones of oxygen depletion within the substrates, and these zones will fluctuate dependent upon the factors mentioned (for example the substrate may become much more widely anoxic at night, when the tank water is less highly oxygenated.) If you have a substance with a lot of surfaces that the microbial biofilm can colonise, over time the lower layers of the film will be composed of anaerobes and facultative anaerobes.
This is from the Wikipedia article on trickle filters for waste water treatment
http://en.wikipedia.org/wiki/Trickling_filter, but it is exactly the same concept. We are now in realms of the "redox value" of the substrate, and it is these "reduction and oxidation reactions" that regulate nutrient availability within the substrate.
I have been under the impression that creating anaerobic areas to convert NO3 to N2 gas is rare and tricky to achieve in freshwater aquariums, usually causing more bother than it's worth.
This is using the coil "de-nitrifier" shown in "Foxfish's" image. In these oxygen rich water is forced to spiral down through the layers of plastic coil tubing into the center of the cylinder. As the water level increases within the body of the unit, the biological media ("bioballs" or similar) become host to bacteria. Th idea is that the O2 is consumed by the aerobic filter bacteria, but somewhere down the coil the O2 is exhausted and facultative anaerobic bacteria develop denitrifying the NO3 into NO and eventually de-gassed as N2. So realistically it is a re-circulating Winogradsky column, the problem comes in that it has to be cycled, to stratify the bacteria, and it is quite tricky to get the right water flow.
The other possibility is the freshwater plenum. The idea of encouraging this bacterial community and applying to planted freshwater aquaria the reefkeepers' "Jaubert" system, with a low-oxygen area in the lower substrate (the "plenum") where de-nitrification proceeds, usually it uses a chemically inert ceramic-glass-foam material "Cell-Pore." Cell-Pore has very small interconnected pores of 50-400 microns. If you have a deep, relatively un-disturbed substrate you will have a de-facto plenum anyway.
And I see there is now another comment by "Brenmuk", I think all the points you make are probably correct.
cheers Darrel