Zeiro,
I have experimented for a couple years with soil substrates (4 different tank builds) and it is very doable.
Do not use potting soil. It is laden with tons of organic material that will go anaerobic and cause problems later. There are aquarium soils, but I don't find them worth the expense. I have gotten good results with the following method.
I buy a heavily digested compost derived top soil (costs about $3 for a 20lb bag). Rip open a corner of the bag to see what you're buying before you do. Do NOT buy if the soil:
- is mulchy and black
- is noticeably wet
- contains perlite chunks or fillers
- contains fertilizer beads
- is warm to the touch
- is derived from manure
- is "enriched" or "fortified" in any way
The soil you buy needs to have a slightly sandy, loamy consistency. Chunks should crumble when squeezed (don't want high clay content). I ended up buying 2x the volume I estimated I needed for a 1" layer of soil. Some of the soil will be lost in the next step.
Next I get a noodle/vegetable wire strainer with a relatively tight wire weave. Like this:
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I scoop a couple cups of the raw soil into the strainer (it needs to be relatively dry) and sift it through into a tub or cooler. The soil coming through should be fairly uniform in texture and should not contain any wood chips, grass or other organics. It should look like dark sand and have a clean, fluffy feel to it. When you get down to just big chunks, wood, etc, throw it away and start again with a new load. This will take a while, but the results are worth it.
Continue sifting until you have enough soil for your project. The resulting soil should look like very dark, moist sand. Run your fingers through it and search again for any obvious undigested organics. Re-sift if necessary. Now wet the soil just enough to make a thick, moist mud. This will eliminate air pockets when you start scooping it in.
Next, line the edge of your tank with the top gravel you want (I use rinsed Flourite), making a dam all the way round the inner edge of the tank. I would recommend against play sand both because I don't like it and I tended to get a lot of hard-to-clean algae when using it with a soil underlayer. Scoop your soil into the well until it is level with your gravel edging. Cover the soil layer with another 1-1.5 inches of gravel (careful not to splash or disturb the mud...scoop gently). Place tank and fill slowly so as not to disturb substrate.
Plant the tank heavily from the beginning. Start your cycle process. You should expect to get a moderate green water bloom a few weeks after starting. This has almost always happened for me when using soil substrate, but provided you can get your plants rooted into the soil in this time, you'll find the plants start to dominate as they can take advantage of the soil nutrition. Some UV sterilization and decreased lighting will get things under control in short order. When the bloom subsides, keep a close eye on your ammonia/NO2 levels and give your cycle time to re-stabilize.
Note that you will need to be -very- careful if you want to relocate rooting plants once things get established. You really do not want to get underlayer soil stirred up once your plants start digging in. If you are going to plant large root system plants like swords, make sure you are never going to move them.
Given a little common sense and caution, I don't see any reason to be scared of soil substrates. I am sure other more experienced users will have good advice as well. Good luck!
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