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Re: Experimental Design: Aquatic Plants

There are a lot of questions about dosing levels for nutrients such 
as nitrate, phosphate, and potassium.  How high should one maintain 
levels in the aquarium?  Tissue analysis of the plants would be a 
good way to see how much of the nutrients the plants are actually 
getting and whether or not they are anywhere near to being deficient. 
Plants are capable of considerable luxury consumption for nitrogen 
and phosphorus, and I am pretty sure for potassium, too.  This means 
that they can store higher concentrations of the nutrient in their 
tissues than they need for maximum growth---that is, growth 
unrestricted by a need for the nutrient.  For nitrogen aquatic plants 
have a luxury consumption range from about 1.5% to a little over 4% 
(%N of dry weight).  For phosphorus, the luxury consumption range is 
from around 0.15% all the way to a little over 0.7% of dry weight.  I 
don't know what the values are for potassium, but they can be found 
in the literature.  If you pull plants out of the aquarium and get 
values at the low end of the luxury consumption range, then more 
dosing is called for.  Highly N-deficient aquatic plants go down to 
about 0.7% N or a little less, and Highly P-deficient plants go down 
to between 08% and 0.1% P.  The point of all this is that you can 
pull a plant out of an aquarium or a lake, analyze it, and get a good 
idea of how much of the nutrient it is getting and whether it is 
deficient, near deficiency, or way off in the luxury consumption 
range so that it has enough stored to allow doubling or tripling its 

So.......If you have access to equipment that can be used for tissue 
analysis, you could answer a lot of levels about dosing levels. 
There are lots of questions that can be asked.  One that I would 
particularly like to find out is: Does low or moderate level iron 
deficiency interfere with nitrogen and or potassium uptake?  I've got 
a suspicion it does.
  Paul Krombholz in dried-out central Mississippi.