Aquasoil basics

[Sebaquafly1]

Hi all,
I also had a question regarding the Cation Exchange Capacity (CEC ) of aquasoil that might fit this thread.

The CEC is sometimes described as a 'nutrient magnet' to make it simpeler to understand. I was wondering how this relates to the buffering capacity of the peat in the soil with regards to KH or that this is more related to the clay particles. For instance, new aquasoil absorbs some of the available PO4 from the water column. This seems related to the CEC of aquasoil, where the aquasoil could apparently become saturated over time. When this happens, does that also mean that the buffering capacity of the aquasoil/peat to absorb carbonates (KH) is lost? If so, does this also happen vice versa?

The reason why I am asking is that I use RO water and could technically chose to keep the KH above 0 (for instance 2 or 3). This means that the peat will react with the available carbonates. However, if this means that the CEC of the aquasoil is also more negatively affected over time, then I might be more reluctant to do that and rather keep the CEC for actual nutrition absorption.

Also, if the aqua soil is saturated with PO4 does that mean it cannot absorb other nutrients such as ammoniacal nitrogen?

 

[Sergey_PAT]

Hi all,
I also had a question regarding the Cation Exchange Capacity (CEC) of aquasoil that might fit this thread.

I’ve been maintaining aquascapes for 4 years now, using soil for cactus or garden topsoil from my plot. On top, I add a cap of neutral substrate. I keep Crystal Red shrimp and Taiwan Bee shrimp, which require water with KH = 0, GH = 6, and pH < 6.



Here’s my reasoning:

1. Shrimp soil typically lasts from 6 months to 1–1.5 years. After that, the aquarium needs to be restarted with fresh soil, because the old soil stops keep pH < 6.
   So, to answer your question: used soil cannot be “recharged.”
2. CES (Cation Exchange Capacity) of the soil is a key parameter. You can find a lot of detailed information about it in scientific literature on agricultural soils. I’d recommend checking it out.

Cation Exchange Capacity also matters in wastewater treatment and process water treatment, where ion‑exchange resins are used in ion‑exchange filters. Here’s an excerpt from a Dupont manual for an ion‑exchange resin.



I might be mistaken, but from this document it seems that different ions have varying affinities for the ion‑exchange resin. According to the table, before the soil can absorb PO₄, it will first take up iodine, sulfates, nitrates, and nitrites — and only then phosphates. By the way - same for aquarium phosphate remover media.

1. In an aquarium, soil is used to lower pH and reduce KH. At least, that’s what the older packaging of DENNERLE Scaper’s Soil and Dennerle Shrimp King Soil states (you can google images to check).

Therefore, if you keep KH > 0 in the tank, the buffering capacity of the soil (in water treatment terms, this is called “ion‑exchange capacity”) depletes faster, because the resin’s ions are consumed binding hardness ions.

1. When people say “soil absorbs phosphates,” it doesn’t always mean true ion exchange is happening. I’m pointing out that phosphates can shift between dissolved and undissolved forms depending on pH.

Here’s a graph showing phosphate precipitation/dissolution depending on pH, from the article Healing the Earth by Healing the Waters: Recycling Waste Nutrients Using Biochar and Limestone (by Kirk D. S. Jones).



I’ve never tried this, but I just thought of something: the phosphate concentration you measure with tests might depend on when you take the measurement. Late in the day, pH rises due to excess oxygen, and phosphates precipitate. By early morning, pH drops, phosphates dissolve, and test readings show a higher level.