Join Date: Feb 2012
Location: South east asia
ADA adds quite abit of nitrates/ammonia, the low pH environment it creates also allows plants to absorb iron more easily.
Personally I use mixed substrates in almost all my tanks; using a laterite or red clay base, followed by top soil and capped with ADA/other artificial substrates. Using exotic substrates may give higher success rates with difficult plants. Having a low pH soil (something most people on the forum will caution you about) actually frees up iron much more readily to be absorbed by the plant roots; you can observe this effect through the glass when you see plant roots becoming red overtime. Some picky plants like pogostemon stellatus color up much more easily. Even Blxya japonica takes on a reddish hue in a rich substrate coupled with high lighting.
That being said, selecting a working combination takes testing and experience. Mixing acidic substrates and calcium heavy soils give weird CA/MG balances that seem to both affect plant growth and algae. In earlier years I've mixed ADA with Seachem's Onyx's sand, the later has a slight buffering capacity and the former is acidic; the results were terrible. Having overly acidic substrates without enough healthy plant growth may result in production of hydrogen sufide or other toxic gases.
All being said, I'd have to agree with OVT that you'd be better off doing either AS or dirt rather than mixing, cos of all the potential factors that could go wrong and separating the layers to reuse is very troublesome. If you want to experiment; at least test these few variables; pH of each soil, availability of carbonates, calcium and iron. Don't mix soils with a great difference in pH. Be aware of iron rich substrates in low pH soils and acidic substrates with calcium or carbonate rich soils.
A simple solution to add nutrients to soils is using root tabs around heavy feeders.