# Potassium question



## Zapins (Jan 7, 2006)

KCl's solubility in water is 344 g/L (20°C), so at tank temperature it would dissolve a bit more than that. In other words it is very soluble and would probably take only a few seconds to completely dissolve, not a few days.

I don't think you need to add dolomite or KCl to get usable soil. I think it probably won't hurt, but as for actual benefit? I doubt it.

Sometimes you need to add potassium to the water column as time goes on, not all soils have exactly the same properties and therefore release potassium at different rates. It is entirely possible to have potassium deficiencies in a soil based tank, though it isn't really that likely - at least after the first few weeks following planting. Once the plants have established root systems they can usually find enough to keep themselves healthy.

K2SO4 (potassium sulfate) is less soluble than KCl, though not by that much - 111 g/L (20 °C). Many people use it as a source of K since it adds twice as much K per anion than KCl does.


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## Greystoke (Jul 24, 2010)

With a few exceptions, all Potassium salt are water soluble, and although some substrate appears to be able to bind Potassium, my experience is that KCl still washes out at the slightest provocation. If anything is left in the substrate, it must be of little consequence.

A Dolomite dusting is fine, as is laterite and some bone meal, but - I think - you should forget the N and the K's. Just occasionally dose some saltpetre (KNO₃).


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## lochaber (Jan 23, 2012)

thanks for the responses, I appreciate it.

I'm bad at planning, and started a tear-down of a tank that wasn't quite working out last weekend, and planned to do a new set-up at the same time. I'm really good at procrastination.  was looking at doing the new set-up tomorrow night, and going over a list of last-minute stuff to pick up, and this was one of them, and after not finding it, I began to question how useful it was.

On a sorta visceral level, adding dolomite to the base kinda makes sense, as dolomite is one of the less soluble of the carbonates, it can help buffer the soil/base, and combined with the low solubility and being under all that soil/substrate, will likely have a minimal effect on the water column. I just couldn't see something as soluble as KCl being worth it.

I might consider bone meal, I'm assuming that's high in calcium phosphate (apatite, I think?), and I think Walstad made a claim that K is better picked up through the water column, while P is better picked up via roots...

I'm not terribly concerned about N, as I figure that will come from fish waste (one of the great reasons to go planted  ), but I'll try and keep it in mind if I seem to be having problems later in regards to K.

thanks again


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## dzega (Apr 22, 2013)

its a hobby. thats why people experiment and try out different kind of things. including substrate preparation. i bet 99%(if not all 100%) in this hobby have added something unneeded to substrate just to see of it changes anything without deeper knowledge in physics behind it.


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## Zapins (Jan 7, 2006)

The issue with dolomite is that it tends to hold onto the nutrients it has and not release them. Also, plant roots can't really access most of the nutrients in dolomite since it doesn't have a good surface area or charge. It is the same situation you see in out door gardening. If you have a very clay rich soil then your plants don't grow well and have a lot more deficiencies because the clay holds onto the nutrients (among other problems). Adding cations like calcium/potassium/magnesium can help release nutrients stored in clay rich soils though. That is probably why aquasoil requires the addition of potassium after a few months.

The good thing about potassium is it is actually pretty non-toxic to plants, so you can add it to the water column without worrying about over doing it. And as stated above it will help release stored nutrients from your soil over time.


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## Greystoke (Jul 24, 2010)

Agreed, but the pH has something to do with it as well. At the lower pH levels cations become more available.


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## Zapins (Jan 7, 2006)

Greystoke said:


> Agreed, but the pH has something to do with it as well. At the lower pH levels cations become more available.


A good point.

Adding CO2 should keep the pH below 7 and make them more available.


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## plantbrain (Dec 15, 2003)

lochaber said:


> So, I've seen quite a few guides about setting up a dirted tank encouraging starting with a dusting of dolomite and muriate of potash.
> 
> I've been wondering about this, isn't muriate of potash (pretty much KCl, real similar properties to NaCl (table salt)) really soluble? would there even be any left in the tank after a few water changes? I imagine it's soluble enough that after a week or so it would have diffused throughout the tank (even if it was put in under all the substrate), and be effectively eliminated after a few water changes.
> 
> ...


That assumes that all the K+ is bioavailable.
In reality, no soil will give up every last bit of the fert. 
K+ is bound in clay in soils and can be tough to remove. K+ is almost always taken up from the water column.

So dose a little to the water every so often. 
Do that, and it'll last forever.


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## skanderson (Jul 25, 2010)

i was considering adding greensand to the bottom of my tank before the dirt goes in to try for a more slow release source of k+. anyone ever try this?


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## lochaber (Jan 23, 2012)

plantbrain> Thanks, I think the bioavailability thing is a good point.

skanderson> I'm not at all familiar with it, I had to wiki it. I have no idea, but I think it's worth looking into. It's one of those things we should keep in mind next time someone asks for a science project. Have them set up a few tanks, some with dirt, some with dirt + green sand, and some with dirt + something else, everything else (equipment, substrate, plants, bioload, etc.) the same, and see if there is any noticeable difference.

For anyone who cares, I went ahead and set up my tank last night (that was a long day...) Just a 10 gallon, some driftwood on slate (I think that tends to turn out better if you do the drilling and such while sober (or at least, not as drunk as I was...)), a bit of dolomite, ~1/2" - 1" of sifted MGOCPM, and another ~1" or so of flourite black sand. 

broke up a bunch of marsilea and planted that all over, a single anubius on the driftwood, a couple crypts in the back corner, and some rotala? (I think? I don't know...) planted horizontal-like along the back, hoping that will encourage a bunch of vertical buds.

Thanks to everyone who responded, I appreciate the input/feedback.


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