[APD] Potassium problems

[Roger Miller <roger at spinn_net>]

I've followed reported problems with potassium for quite a while and I've 
built up some background on it.  I may have even had some of those problems - 
or similar problems myself.  The information I have seen comes exclusively 
from agricultural, hydroponic and horticultural sources.  Many of our 
"aquatic" plants are closely related to plants in common agricutural and 
(more often) horticultural use so I have reason to believe that information 
from  those sources should apply to aquatic plants.  The exception is that 
mechanisms involving potassium uptake from soil through the roots should may 
not apply to foliar uptake.

On the basis of my reading I think a few things are indicated.

1) Potassium at reasonable concentrations (into the hundreds of ppm) is not 
toxic to plants.

2) There is an antagonistic relationship between the four common alkali and 
alkaline earth elements such that a large excess of one element can cause 
deficiencies in another.  The published sequence is (as I recall)

K > Na > Mg > Ca

I'm working from memory so the order of Ca and Mg in the sequence could be 
wrong.

Where elements on to the left in the sequence are more likely to antagonize 
elements on the right.  This sequence was originally published in (as I 
recall) Science in or about 1934.  It is well founded and widely recognized.  

The mechanism for the antagonism is not simple.  In the specific case of K 
and Ca, there appear to be problems both at the root-soil interface and with 
transport within the plant.  Information on the mechanism has proven very 
difficult to come by.   I have no reason to believe that the mechanisms are 
the same in every case or that the mechanisms are simple in any case.

3) Common recommendations are that in order to avoid antagonistic effects K 
should not exceed Ca , but some sources recommend that K can easily be 2x 
calcium concentrations without problems.

4) The sensitivity of different plants to antagonistic effects is highly 
variable.  Furthermore, there are a number of other contributing factors that 
may make the occurance of problems difficult to predict under normal aquarium 
conditions.  I can think of:

   a) Actual (not estimated) potassium concentrations.  There are very few 
measurements of potassium concentrations available.

   b) Ca:Mg ratios in measured hardness and the actual concentrations of Ca 
and Mg.

   c) The nature and composition of the substrate

   d) The concentration of sodium in the aquarium water. 

I'm sure there are other factors.

Given what I have read, I would not expect potassium:calcium antagonism to 
set in unless water was soft.  For instance, if the aquarium water has 5 dGH 
of calcium hardness (about 35 ppm calcium) then there should be *no* 
antagonism unless the potassium concentration was well in excess of 70 ppm.  
In the case where someone has hard water (say 20 dGH with normal Ca:Mg 
ratios) potassium:calcium antagonism should not occur in fresh water. In 
particular, with the experimental conditions that Tom is using to test for 
potassium effects on Ammania, I would expect that there should be no problem.

I am not saying that the problems report to be cured by stopping or reducing 
potassium doses were not caused by potassium:calcium antagonism but in most 
cases people need to look for different cause:effect relationships and maybe 
they need to question their own observations.

We went through this on the Aquabotanic list months ago and from that I 
finally concluded that in most cases either;  potassium concentrations were 
originally *much* higher than people estimated they were; the symptoms 
reported as cured were *not* calcium deficiency symptoms or; the observations 
were too non-specific to be useful, regardless of how convinced the observer 
was.

I also think that there have been a few good observations of real problems 
that probably are potassium:calcium antagonism.  Because of those cases and 
because plants don't need particularly high potassium concentrations for 
healthy growth I think it is advisable to be pretty conservative when dosing 
potassium.

Intenet forums (all of them, as near as I can tell) are pretty strange when 
it comes to things like this.  When a new mechanism comes up that might cause 
problems some people over react.  They assign problems to that mechanism even 
when there is no indication for it.  Some people even invent problems so they 
can jump on the bandwagon.  It's all a little weird.


Roger Miller
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