Should you siphon your aquasoil substrate? How to maintain aquasoil substrates?

Do substrates/aquasoils needs maintaining?

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.

Organic detritus is made of partially broken down old leaves and livestock waste. While it does release some nutrients to the substrate zone over time, a large accumulation of this waste interferes with root formation. Delicate plants prefer a cleaner substrate, as counterintuitive as the idea of a 'clean substrate' sounds.  

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 ammoniacal nitrogen and this provides a rich source of Nitrogen for rooted 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 nitrogen 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.

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.

Checking plant roots during replanting cycles can be an indication of substrate health. Blackened and or mushy roots can indicate substrates that have gone too long without servicing. 

Clearing build up of organic material

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. We call this Water Change the 2Hr Way. 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.

Enriching substrate with root tabs/aquasoil

There are two main ways to enrich aquasoils that have depleted. The first is by mixing in rich new ammonia rich aquasoil every once in a while (every 3 to 6 months is a good rate). The other method is by using nitrogen rich root tabs. Our 2hr Aquarist APT Jazz root tabs are designed to be rich in slow release ammoniacial nitrogen for this exact purpose.

Nutrients like Potassium and Magnesium are easily absorbed through the leaves for aquatic plants whereas having ammoniacal 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 contain ammoniacal nitrogen are much more effective than if the nitrogen source was nitrate based. Terrestrial based slow fertilizers may also contain trace mixes that are too heavy or have the wrong ratios, so if using terrestrial based slow release fertilizers, stay away from the ones that contain trace mixes.

This tank at the 2hr Aquarist gallery has been maintained for more than a year using the same aquasoil substrate. The stem plants have been trimmed and replanting for many cycles with no change in substrate. Consistent maintenance and enrichment of the substrate makes it look like each new growth cycle was planted in fresh aquasoil.

Head here to read on more substrate topics

Head here to read more on water column vs substrate fertilization