Aquasoils are now standard in many planted tank setups, but there are very few sources if any, that cover how to maintain aquasoil substrates over the long run (months/years). Many of the concepts here will apply to inert substrate tanks as well. For slow growth tanks as well as for folks that tear down their tanks yearly, it may not be obvious whether the substrate is well maintained or not. However, substrate problems impact many folks - and it affects plant growth and overall tank stability, in a fashion that cannot be compensated purely by managing water parameters/fertilizers.
Indeed many of the crowd that tear down their tanks yearly do so because they start to run into persistent plant health issues after months of not touching their substrate. The other group of folks for which maintenance of substrate has large impact are the folks that are attempting to cultivate delicate/rare/small sized species which are more sensitive. Tissue culture plants and small carpets melt more easily and get algae issues more easily when there is detritus build up in the substrate.
Hygrophila sp. Chai is a delicate stem plant; it is very vulnerable to algae and you need a clean substrate to grow well formed specimens.
A well maintained aquasoil tank need not be tore-down completely for many years. There are two main things that happen over a longer time horizon that is not immediately visible to most aquarists.
The first is the depletion of nutrients as well as buffering capacity of the substrate. Many aquasoils are enriched with ammoniacial nitrogen and this provides a rich source of Nitrogen for rooted plants. Ammonium is an especially effective nitrogen source for many species and they grow fuller with better form when it is present in the substrate. Ammonium binds to soil, and so you need aquasoil/soil to have it available to plants. This is why Aquasoil substrates always grow plants better than just inert substrates spiked with root tabs. Laboratory analysis of aged aquasoil samples from Tom barr's tanks (Barr report Vol5, issue 1) shows that while ammonium levels are depleted over time, P, K and Fe levels of aged soils were actually higher than in new aquasoils. This may largely be due to the rich water column nutrient dosing done in that particular tank, demonstrating that water column nutrients do make their way into the substrate over time. Depending on the amount/type of soil used, water change schedules and growth cycles, aquasoils start depleting their nutrient stores significantly after 6 to 10 months. In soft water tanks, their buffering capacity may last awhile longer than that.
Many species such as Eriocaulon quinquangulare and Blood vomit grows faster and more stable with ammonia rich aquasoils. Many stem plant species do better as well, even though they have smaller root systems.
The second aspect is that waste organic material builds up in the substrate over time. This comes from old decaying old roots as well as breakdown of organic debris (such as old leaves) in the tank environment. This organic material buildup has a tremendous impact on delicate, smaller plants and their root systems. Tougher larger plants tend to be less affected. This organic build up leads to poor growth, algae and melting of lower stems. Whether the cause is due to overly labile conditions or having an overactive decomposer microbial community is not entirely clear.
What is clear though is that clearing up build up of organic detritus leads to observably better growth, less algae and more stable growth for delicate species. Newer aquarists thinking that having some organic detritus build up supplies nutrients to rooted plants are not entirely wrong, as waste does add ammonia and other nutrients to the substrate. However, having a large amount build up has largely negative consequences. Given that most soils/aquasoils will have more than adequate nutrients for plants, it is never necessary or conducive to allow detritus to build up in the substrate. In fact the opposite is true, planted tanks grow better with less algae when the substrate is keep 'clean'. Build up of organic detritus is one of the key factors plaguing folks facing persistent BBA issues.
Example of tank with a lot of organic detritus. Delicate plants will not grow well in this no matter the tank parameters, and this is an invitation to algae in the long run.
Organic detritus builds up very quickly in tanks where there is deteriorating plants. This aspect makes it doubly penalizing for beginners who face problems tuning their tanks right from the start. Then as detritus builds up in the substrate due to the first round of melting plants, it gets harder to grow the next round of plants and algae problems start appearing etc. Folks that do not know how to deal with this often end up tearing down the tank and restarting afresh. Such restarts are really not necessary; just clearing up the organic detritus from the top layer would work.
If you run a very high light tank with fast growth rates or high bio-load, which usually means higher amounts of detritus being produced in an environment where algae is easily triggered, doing light substrate surface vacuuming every week as part of the water change schedule is important. For slower growing tanks with low bio-load this can be done less frequently; once every 2 or 3 weeks or so.
To do it; hover a siphon over the surface of the substrate while kicking up surface detritus with a turkey baster. For weekly cleaning, you should not aim to disturb more than just the top 1cm of aquasoil or so.
A couple of videos that demonstrate this:
Deeper cleaning once in a long while; this is something that should be done for tanks that have huge amount of old roots (because of dense planting) or aquasoil substrates that have been used for years. This can only be done after plants have been all uprooted, so usually it is done with replanting cycles for plants. It is the same as above except the aim is to get detritus out of deeper layers. There is no particularly delicate technique to do this; you can move the aquasoil with a small spade or your fingers while siphoning up smaller particles and detritus, this goes with manually clearing out old roots & portions of old growth trapped in the soil. This would usually be accompanied by a large water, as stirring up deep layers of substrate usually release ammonia and labile organics into the water column.
This is also a good time to add fresh ammonia rich aquasoil or ammonia based root tabs to enrich the soil.
There are two main ways to enrich aquasoils that have depleted. The first is by mixing in rich new ammonium rich aquasoil every once in a while (small amount every 3 to 6 months is a good rate).
As mentioned above, one of the aspects of aquasoils is that ammonium binds to soils which makes it an excellent nitrogen source for growing plants. Having it available grows plants fuller with better form when it is present. Nutrients like Potassium and Magnesium are easily absorb through the leaves for aquatic plants where as having ammoniacial nitrogen in the substrate is advantageous. If dosed into the water column instead, ammonia would quickly be oxidized to Nitrates in the water column and may also trigger algae. Nitrates on the other hand, don't readily bind to soil - so having them in root tabs just mean that they leech out slowly and contribute to NO3 levels in the water column.
To this end, root fertilizers that are ammonium based are much more effective than if the Nitrogen source was nitrate based. Many terrestrial based slow fertilizers may also contain trace mixes that are too heavy or have the wrong ratio. Thus, I would recommend root tabs that do not contain trace elements unless you know that the trace element mix has been calibrated carefully for sustained used in aquarium substrates (unlikey to be, in most cases). Furthermore, trace elements are easily added in through water column dosing, and most aquasoils have good levels of it. If you use stuff like osmocote, insert individual balls beneath plants every 3 to 4 months; bury them as deep as possible. Aim to not uproot such areas in the coming weeks.