Bubbles are pretty normal in Walstad tanks- it's just normal decomposition. It's when they smell like sulfur that they are anaerobic- this is more uncommon. boopsie, if your soil had anaerobic pockets you would have been smacked in the face by rotten egg odor when the bubbles came out. Just keep poking the soil once a week, and you'll be fine.
VJM, I have only seen maybe one or two people say they've actually had anaerobic pockets, so I suppose it can happen. But I am beginning to suspect that this threat has been rather exaggerated. The pink Christina Aguilera monsters of the aquarium hobby.
I still poke my substrate, though, even with my MTS.
One of my tanks has flourite mixed with the soil, and the other has plain Hartz pH 7 clay kitty litter (non-clumping, non-scented) in the soil. But, as far as I know, the purpose of these (or any clay mixed with soil) is to provide CEC (cation exchange capacity). The clay is able to store the nutrients that are being released by the decomposing soil, thereby keeping those nutrients within the soil and available to plant roots. As a bonus, the CEC of clay keeps those nutrients from being released into the water column.
EDIT: Even with these additions, I still get bubbles. But not the bad bubbles.
EDIT #2: Not sure if Gallium mentioned this, but a good rule to follow to avoid the formation of anaerobic pockets is to make sure that the soil layer is only 1-1 1/2 inches deep. Too deep substrate is said to be a cause of anaerobic pockets.
I'm wondering how clay or an inert substance like sand would help avoid anaerobic pockets?
the bubble that come out are nitrogen gas, that has been fixed by nitrogen fixing bacteria, which is a process that's anaerobic, nitogrn gas is toxic even to us mamals unless it has been "fixed" by the nitrogen fixing bacteria, I have not checked in a few years to see if they have solved this "fixing" process, it was an unknown
@Nicci you can not avoid nature look up nitrogen fixing bacteria it will explain the process, it is basically how things are broken down so plants can use them, ammonia nitrites and so on, it's kind of the nitrogen cycle that happens in soil/dirt if you sit on the shore of a lake/pond/swamp toy will see bubbles coming up from the soil, it's just a part of nature
@Stone- Um, I think possibly you misunderstood me? I don't mind the bubbles at all, at least not the ones that come from normal decomposition, or bacterial denitrogenation if you will. ;) These bubbles are the ones that release nitrogen.
Actually, anaerobic pockets release hydrogen sulfide, not nitrogen. This is what is toxic to fish, and why it smells like sulfur. There is always some anaerobic activity occurring in the soil, but a buildup of anaerobic decay in a pocket where it gets trapped results in an accumulation of hydrogen sulfide. Usually plant roots are enough to supply oxygen to the soil- anaerobic bacteria can only live in oxygen depleted areas, and so where there is oxygen there are no anaerobic pockets. This is why a substrate that is too deep can cause this problem.