nutrient IV drips?

[Paul Krombholz <krombhol at teclink_net>]

I think the opinion pendulum is swinging way too far in the direction 
of too frequent additions of nutrients that can actually be added 
much less frequently.  Let's look at the various groups of nutrients 
and see if we can determine which ones have to be added frequently, 
and which do not.

Why would we have to add a nutrient frequently?  Here are some 
possible reasons:
(1)  The nutrient has only a narrow range of concentrations where the 
plant is neither harmed nor deficient.  Harm to other tank 
inhabitants is also a concern.  Here, I am thinking of ammonia and 
urea, which become toxic at about the same levels.  However, we don't 
have to add nitrogen in these forms.  We can use nitrate, which is 
much less toxic at high levels.
(2)   The nutrient does not stay in solution, but precipitates out 
and becomes unavailable.  Iron is the one I am thinking of here. 
Soluble ferrous compounds get oxidized to insoluble ferric compounds 
in only a day or two in the presence of oxygen.  Iron EDTA lasts 
somewhat longer, but the chelating agent, EDTA, is sensitive to light 
and gets broken down.  Iron DPTA is the best, lasting for one to two 
weeks.
(3)  (very similar to #2)  The nutrient can get converted chemically 
to an unavailable form.  Here, I am thinking of nitrate, which can 
get converted by the process of denitrification to N2, which is not a 
precipitate, but still unavailable.


That's all I can think of.

Let's look at the micronutrients, including iron.  All of them have a 
wide range between toxicity and deficiency.  It is possible to put in 
enough so that a small volume of solution can grow a rather large 
volume of plants.  In mixing up nutrient solutions for hydroponics, 
it is routine to add the micronutrients only at the beginning.  The 
solution is discarded when the macronutrients are all used up.  It is 
not considered necessary to freshen up the micronutrients while the 
plant is growing.

Looking at the macronutrients, if you avoid ammonia or urea, there is 
also a wide range between toxicity and deficiency.  There is a slight 
concern with phosphate because, if you get it too high, it starts to 
precipitate out soluble iron.  You also have to keep the pH from 
getting real high, or the phosphate will precipitate out with 
calcium.  CO2 additions come in handy here.  A concern with nitrate 
is that it can be lost by denitrification and not all get to the 
plants.  Basically, you can add enough of all the macrontrients to 
support growth for quite a long time.

I conclude that Nitrogen and iron are the  two elements that you can 
not add in excess and expect that they will hang around until they 
are used up by the plants.  Well, to be picky, I suppose that calcium 
can go into snail shells instead of the plants, but old snail shells 
limestone, etc. release it back. It is not much of a problem to keep 
calcium levels up.   Nitrate seems to go somewhere else if the plants 
don't take it up.  You really can't expect it to hang around and 
eventually all go into the plants.  That is why nitrate testing is so 
important for the aquatic gardener.  Iron, also, doesn't wait 
patiently for the plants to get it.  As the chelating agent gets 
broken down by microorganisms or light, iron precipitates out.  If 
there are not anaerobic areas in the gravel where the precipitated 
iron can be reduced and made soluble, it will remain unavailable.

My experience with iron DPTA is that it lasts in solution up to three 
weeks. It would be better if the commercial solutions did not include 
iron with all the other micronutrients, because most people add these 
solutions on a weekly basis to keep their iron up, and they are 
adding unnecessary amounts of the other micronutrients.  It would be 
better if you could get a separate iron solution and a 
one-shot-between-water-changes solution for the other micronutrients. 
I like to add nitrate every one or two weeks.  All the other 
nutrients can be added in quantities sufficient to last for months.







-- 
Paul Krombholz in well-watered Central Mississippi, where we got 
drenched again, today.  (2.55 inches!)