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Re: JoAnn's plant nutrient experiments


I have some thoughts on your aquatic plant nutrient experiment 
($0.05 worth) (thoughts #2-#6).
Apologies for length to everyone else.  
[The archives  <http://fins.actwin.com/aquatic-plants/index.php> 
let you skip it! hooray for the archives.] 

>2.  . . . substitute for potassium nitrate, so . . . no-potassium solution?

Use sodium nitrate. Sodium at that concentration is far in excess of any 
nutrient needs (I think tap water is, too!), yet also won't hurt the plants. 
Ammonium nitrate, on the other hand, would add lots of extra nitrogen. 
(Then you'd need to add a "+ ammonium sulfate" treatment, too, to control 
for the extra ammonium-nitrogen!)  

Just to get really fine-tuned: to get the same concentration of IONS as if you 
used potassium nitrate, use 0.85 g NaNO3 (at 85 g/mol) instead of 1.0 g of 
KNO3 (101 g/mol). (I notice the other treatments don't make this minor 

Finally, nitrate salts are all pretty reactive. Be moderately careful.

>3.  Are the solutions . . . practical? 

Stock solutions involve a little more arithmetic at first, but save a lot of 
weighing & mixing later.
The idea is to make a concentrated solution of a nutrient, and add a 
measured amount of that to your 2L nutrient bath.  It's a lot easier to just add, 
say, 10 mL of each nutrient stock solution, than to repetitively weigh out each 
the chemicals separately for each "tank." 

Plastic measuring spoons for giving liquid medicine to babies are really 
handy here, the kind that look like a test tube with a spoon on the open end. 
They should have markings for mL as well as tsp & Tbp. If you want to do 
extra arithmetic, figure it out for tablespoons instead of mL. [Maybe that's 
the math lesson?]  But dividing or multiplying by tens is easy, and it's what 
the metric system is for!

For example, the potassium nitrate solution.  You want a final concentration 
of 1 g in 2L, but you want to dissolve that 1 g in a dose volume of, let's say, 
10 mL of stock solution.  You'll need a lot of 10 mL "doses" so make up maybe 
500 mL. 
You'd need 500 mL divided into doses of 10 mL, times 1 g in each dose, 
or 50 g of potassium nitrate. 
Put the lid on the KNO3 + H2O, and shake till dissolved.

Us geeks call this kind of single dose an "aliquot," which we also use as a verb 

>4.  . . . Egeria densa . . . ?

Egeria sounds ideal. Not only because it grows fast so you can measure 
differences quickly. But also because growing fast means it runs out of 
reserves quickly -- so you can see differences quickly!  Would you measure 
growth by length, number of leaves, weight-when-blotted-dry ?

>5.  Wouldn't we develop algae problems? . . .

Sterilize (or nearly sterilize) the plants (use the archives to find out safe + 
useful concentrations for permanganate, or bleach, or H2O2/peroxide);  
sterilize the nutrient solutions and the containers (boiling both at once in the microwave uses less clean-up and less electricity than the stove);  
use fast-growing plants (yet another reason!). 
You may get some algae from the air, or kids's muddy hands, or 
cross-contamination, or the dog's drooly tennis ball (these things happen). 
But you can minimize & delay the algae infection. 

>6.  . . . ways to improve this experiment?

Use REPLICATES.  If you have only ONE of each treatment, you won't know 
whether it's the treatment, or that dog drool, or that warm spot by the 
window, etc, that is really responsible for the results you see. 
Chance events can have measurable effects  
-- but it's unlikely that if all (or most of) 3 or 4 of the no-nitrogen tanks turn 
yellow in the same pattern, that it was unrelated to the missing nutrient. 
Added bonus: if you spill or otherwise lose one or a few, you still have some 
results for that treatment. [Usually.] 

Replicating lets you teach the basics of probability & statistics; the ideas that 
chance events matter AND that averaging things out can help you rule out 
chance; and even what fractions & averages really mean. 

(Replication is why I calculated for a 50-dose stock solution: 3 or 4 reps of 
5 or 6 treatments, plus allowance for spills and mis-mixing, makes for a lot of 

I hope this is useful to at least JoAnn!

landlocked in New Hampshire, 
in the Maxfield-Parrish-painting hills complete with just-leafing-out trees & 
steep river valleys & gold-and-rose-edged cumulus clouds.  
it really looks like that here, 10 miles north of where he painted. 
zot at umit.maine.edu