Potassium Cycle

For a high-level tour of how every element moves through the tank, start with Nutrient Cycling. Potassium is the nutrient planted-tank keepers dose by the spoonful and the one whose shortage shows up as stunted, pinholed leaves.

The macronutrient that acts like a trace element

Potassium leads a curiously split life. Chemically it is about the simplest element in the whole model: dissolved potassium is the only form that exists in any aquarium, and it does nothing dramatic — it does not change oxidation state, does not precipitate, does not bind to organic matter, and does not escape into the air. In that sense its chemistry is barely more involved than molybdenum, one of the rarest trace metals.

Biologically it is the exact opposite of a trace element. Potassium is the dominant positive ion inside every living cell — the master regulator of cell water balance, the charge that balances the books when a cell takes up nitrate and ammonium, and an essential helper for dozens of enzymes. Cells carry a great deal of it: a unit of plant tissue holds something like a thousand times more potassium than iron and a hundred thousand times more than molybdenum. It is structural, not catalytic — every gram of new growth comes with a substantial potassium tax.

That combination is exactly why potassium is a true macronutrient despite its simple chemistry. A densely-planted tank running Vallisneria and Cryptocoryne can pull tens of milligrams of potassium out of the water over a few months. Without replenishment from the substrate or from dosing, the plants run their tissue reserves down and visibly stunt — and this is precisely the deficiency that the planted-tank Estimative Index fertilizing routine is built around. Potassium dosing is one of the central levers of the planted-tank hobby, which is why the model tracks it as a full macronutrient rather than a bookkeeping afterthought.

Where potassium lives, and how much

Potassium sits in two places: dissolved in the water column, and dissolved in the pore water between the substrate grains. The water-column pool is what your plants' leaves draw from and what a test kit measures; the pore-water pool is what rooted plants tap through their roots, and it exists only in tanks with a real substrate.

Typical concentrations span a wide range, and that range is the whole story of potassium availability:

Water type	Potassium level
RO / DI refill (no remineralizer)	essentially zero
Soft / forest water	very low
Typical municipal tap	low — a few mg/L
Estimative-Index-dosed planted tank	high — routinely tens of mg/L

A new tank starts with whatever potassium its source water carries — a few milligrams per litre for typical tap, near zero for sterile RO. Pore water starts empty and fills in over the run as the substrate's organic matter breaks down and as dosed potassium works its way down from the water column. RO users with no remineralizer should expect to supply essentially all of their tank's potassium themselves, since a sterile refill brings none.

How potassium limits growth

Every photosynthesizing species in the model carries a potassium gate on its growth, bundled in alongside nitrogen and phosphorus under Liebig's law of the minimum — growth runs at the pace set by whichever of the three is scarcest. Potassium goes into that "minimum" rule rather than dimming photosynthesis directly, and that placement encodes a real biological fact: a potassium-starved plant slows down without losing its green. Unlike iron starvation, which bleaches a plant pale, potassium shortage looks like sluggish, stunted growth with healthy color — often with the characteristic pinholes and yellowing leaf margins aquarists learn to recognize. When potassium hits zero, growth stops and uptake stops with it, which keeps the books balanced exactly the way nitrogen does.

Different groups have different appetites. Ordinary algae have such high-affinity uptake that they only feel potassium limitation at genuinely depleted levels (Healey 1973; Reynolds 2006). Cyanobacteria are tuned tighter still, which gives them an edge in potassium-poor water. Macrophytes, with their bulk tissue demand, run out of headroom soonest — they are the plants that show potassium deficiency first in a real tank (Madsen & Cedergreen 2002).

Rooted plants get a second route. Like their nitrogen and phosphorus uptake, they can draw potassium from either the water through their leaves or the pore water through their roots, and they take the better of the two. A plant rooted in potassium-rich substrate is not starved just because the open water is thin — and vice versa. This dual access, with its tighter root-side affinity, is why heavy root-feeders can thrive in a lean water column as long as the substrate holds a reserve. The exact half-saturation values for each group are tabulated in the Parameter Reference.

How potassium moves through the tank

Potassium runs the simplest loop of any tracked element — no chemistry, just biology shuttling it in and out of living tissue:

Into tissue on growth. Every unit of carbon a plant fixes pulls a proportional amount of potassium from the water (or, for rooted plants, split between water and pore water according to where each can supply it).
Back out on respiration and decay. Maintenance respiration returns a trickle continuously, and when organic matter decomposes — by bacterial decay, by sunlight, by the anaerobic nitrate pathways, or by soil mineralization — it releases its potassium back to dissolved form. In a substrate, that released potassium goes to the pore water; in the open column, to the water.
A surplus pulse on death. Macrophytes and cyanobacteria stockpile potassium above the baseline tissue content — plants in particular hoard it for water-balance regulation (Marschner 2012) — so when they die they release that surplus in a small pulse, leaving their detritus at the ordinary tissue content the decay path expects.
Diffusion between substrate and water. In a planted substrate, potassium diffuses up or down the concentration gradient (Li & Gregory 1974). Dose the water column and some potassium works its way down into the substrate over a day or two; let the substrate leach and it diffuses back up.
Dilution at water changes. A water change dilutes the water-column potassium and tops it back up at the refill water's level. The buried pore-water reserve, sitting below the bulk water, is not diluted — which matches the way a planted substrate holds onto its root-zone potassium through a partial water change.

The per-group tissue ratios behind all of this are listed in the Parameter Reference.

Dosing potassium

Potassium dosing is the bread and butter of the Estimative Index routine, and the model handles it as a simple, direct addition: the dose lands in the water column and steps the concentration up, then drifts back down under uptake, dilution, and refill until the next dose. There is no chelation and no slow-release staging — potassium is ready to use the instant it dissolves.

The common potassium fertilizers — sulphate-of-potash, muriate-of-potash, and saltpetre (potassium nitrate) — all deliver the same potassium to the same pool; only their partner ions differ. That partner matters little for potassium itself: the sulfate that rides along with sulphate-of-potash, for instance, is negligible next to a tank's tap-water sulfur baseline. The one combination worth flagging is potassium nitrate, which also delivers a meaningful dose of nitrate — in the model you configure the potassium and the nitrate sides separately, which is deliberate, since Estimative-Index keepers routinely mix potassium nitrate with sulphate-of-potash in ratios of their own choosing. In a planted substrate, dosed potassium also seeps gradually into the pore water over the following day or two, much as dosed ammonium works its way down to the root zone.

What the model leaves out

A few potassium behaviours are deliberately out of scope, because they would add machinery without changing the outcome for most tanks:

No clay cation-exchange buffering. Real aquasoils carry a large cation-exchange capacity that grabs dosed potassium quickly and releases it slowly — a buffer the model does not yet represent for any nutrient. If a keeper finds that potassium crashes implausibly fast after a water change in an aquasoil tank, this missing buffer is the likely reason.
No potassium toxicity. Very high potassium can stress shrimp and snails by interfering with their calcium balance, but the model has no toxicity response for it. In practice the levels reached by ordinary dosing stay well clear of that range.
No exotic chemistry. Potassium does not form organic complexes worth tracking, does not precipitate on any timescale an aquarium cares about, and does not affect alkalinity — its uptake is balanced by the cell pumping out hydrogen ions, so it leaves pH untouched at the tank scale.

Checking potassium in a run

The simulation reports both the water-column potassium concentration and the pore-water concentration (the latter zero in a bare-bottom tank), along with their totals. The mass-balance diagnostic treats potassium as a first-class element, closing the books against the starting stock plus everything dosed and introduced, minus what water changes carried out; a small positive drift is normal in macrophyte-heavy tanks, reflecting the surplus potassium that plants release as they die. For tracing where potassium is going, the flux and growth-limitation diagnostics will attribute it process-by-process and show potassium limitation ranked alongside nitrogen, phosphorus, light, and carbon for any given plant.

How potassium interacts with the other cycles

Nitrogen — potassium and nitrogen move together inside the cell: potassium is the counter-charge that balances nitrate and ammonium uptake, and rooted plants draw both from water and pore water through the same dual-source logic. Potassium nitrate fertilizer couples the two at the dosing end as well. See Nitrogen Cycle.
Phosphorus — potassium's companion macronutrient in the Liebig minimum; in a fast-growing planted tank, nitrogen, phosphorus, and potassium are the three that take turns being the bottleneck. See Phosphorus Cycle.
Trace metals — potassium reuses the same bookkeeping machinery that tracks the trace metals, even though its quantities are enormously larger. See Micronutrient Cycling.

Key references

Britto, D.T. & Kronzucker, H.J. (2008). Cellular mechanisms of potassium transport in plants. Physiologia Plantarum 133, 637–650.
Healey, F.P. (1973). Inorganic nutrient uptake and deficiency in algae. CRC Critical Reviews in Microbiology 3, 69–113.
Karley, A.J. & White, P.J. (2009). Moving cationic minerals to edible tissues: potassium, magnesium, calcium. Current Opinion in Plant Biology 12, 291–298.
Li, Y.-H. & Gregory, S. (1974). Diffusion of ions in sea water and in deep-sea sediments. Geochimica et Cosmochimica Acta 38, 703–714.
Madsen, T.V. & Cedergreen, N. (2002). Sources of nutrients to rooted submerged macrophytes growing in a nutrient-rich stream. Freshwater Biology 47, 283–291.
Marschner, H. (2012). Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press.
Reynolds, C.S. (2006). The Ecology of Phytoplankton. Cambridge University Press.
Sterner, R.W. & Elser, J.J. (2002). Ecological Stoichiometry: The Biology of Elements from Molecules to the Biosphere. Princeton University Press.