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[APD] Flourite/Porous substrates vs soil
Soil/OM layering produces distinct bands/zones of reduction and bacteria that thrive in these zones.
but............Plant roots like O2 and respire. Plants can either obtain the O2 from the regions around the roots or import the O2 down to the roots. Eitherway, they need the O2.
The other good thing about a porous substrate that allows O2 around the outer surfaces: cycling and remineralization of waste/detrital matter/organic matter. Aerobic bacteria are much faster at this than anaerobic bacteria(roughly 18X faster). So a porous substrate has excellent characteristics for both remineralization of waste back into useful plant available forms and still provides the bacterial mediated anaerobic zones for reduction of trace metals.
What precisely the substrate has in it, I think matters less than these characteristics. This also fits with many observations.
Having the bands/layers of reduction and bacteria protected by the grain and internal spaces like in porous substrates provides the best of both worlds.
There are still the same bands and layers, but they are protected from uprooting yet still accessible to plant root hairs. The outer portions of the grains allows for high O2 levels around most of the roots so to provide the O2 they need rather than to import it via aerenchyma(and the associated cost with transport/void air spaces/lacunae).
Some plants have more advanced/developed aerenchyma in their root systems than others.
Perhaps that is why I have better success with more plant species in my non CO2 tanks than many,
and why the porous substrates tend to do much better with certain plants while others see less significant effects vs say say/laterite etc.
I do add a small amount of OM in every substrate, even marine systems, but I do add a bit more to a non CO2 tank. Since growth is much slower in those tanks and I do not dose anything other than fish food, they stand to gain more during the first few months till they get well established. These CO2 limited growth rates, lower light and lack of dosing are important points.
I will say that the results using porous type substrates definitely enhances the growth of many plant species in non CO2 tanks vs soil+ plain old sand much like the difference between plain sand+ laterite and Flourite for example.
Now for those that are cheap, use the MPV turface+ 30-50% sand(for weight).
I add OM (I think it really does not matter too much where it comes from and what type it is) to help start off the tank, not really for long term supply of macro nutrients.
You can re enrich the tank every few months or year etc if you want, sort of a "repotting".
Horticultural folks do it all the time, and the soil they use does not last indefintiely in terrestrial plants and I would suspect it last even less in an aquatic system due to leeching and diffusion, but the anaerobic nature will help preserve some nutients also. Or you can add jobes spikes/soil "mud cubes", clay balls etc every few months.
I think a total tear down and a repotting of the entire tank would help more. Dosing once every 6-12 months of the macro's if you wish. Personally in higher light/CO2 enriched tanks, I find this a problem. You are going to dose the traces anyways and limited macros are not going to prevent algae, so why bother, add the macros along with the traces. Both will work, does not matter where the macros come from or put, as long as you have them and a steady supply for growth, but there are easier ways that have more control over the nutrient levels.
At lower light, the substrate dosing methods work better and last longer. But many folks are hung up on the more light is better or somehow "required" for redder plants or that Gloss, Hair grass etc "must have" higher light. 2 w/gal has done the trick for most every plant I've had. Good CO2 levels at low light provides great growth but slower and gives great wiggle room for dosing routines, whatever they maybe.
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