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PO4 additions

Roger wrote:

>Plant tissue analyses show a big range in nutrient concentrations.  It
>seems to me that for at least some nutrients there should be a range of
>concentrations where the nutrient is growth-limiting but pathologic
>symptoms of deficiency don't appear.  This is particularly true for
>macronutrients; it seems that plants can adjust their growth rate based on
>the availability of macronutrients and remain healthy over a wide range of

That is my impression as well.  It certainly fits with my observations.  It
does seem to me that phosphate limitation is _less_ likely to cause
pathological symptoms, or at least you can limit it more strictly than
nitrogen.  It seems that in a fish tank, it is easier to put a tank in a
nitrogen limited state (without supplementation of nitrate) than a
phosphate limited state.  A nitrogen limited, phosphate non-limited tank
shows definite signs of trouble, both in terms of algae problems and poor
plant growth.  

If you supplement with nitrate, you can easily induce a phosphate limited
state assuming other macro and micronutrients are in adequate supply.  I
don't think I've ever seen an algae problem caused by phosphate limitation,
and you ahve to starve the tank of PO4 for quite a while before you see
much more response than retarded growth.

Again, I think that limiting K would cause problematic "symptoms", but I
see no reason to limit K, as it is not implicated in algae problems even at
high levels, there is a wide range of acceptable concentration, and it is
easy to dose right along with the nitrate. (My observations had lead me to
the same conclusion as Steve D's test kits quite some time ago... if you
are dosing with KNO3, you'll have an adequate amount of K in your tank)

>For instance, I concluded last fall that some of my tanks were phosphorus
>limited.  I concluded that because when I added a phosphorus-containing
>fertilizer there was a burst of growth that I didn't get when I added the
>other components of the fertilizer separately.  Growth prior to adding the
>fertilizer was fairly slow, but there was nothing unhealthy about the
>appearance of the plants.  Algae didn't become a sudden problem.

That has been my experience as well.

>My point is that as long as there's a range of nutrient concentrations
>that we can work in we *should* be able to have nutrient-limited growth
>without having diseased-looking plants.  

As long as we pick the right nutrient to limit.  I still favor P.  I'm not
sold on Fe, though I can see how it might work if you are _only_ limiting
it in the water column with a plentiful supply in the substrate. (as in the
soil based tanks that Diana and Steve P favor)

This has been an interesting conversation!  Thanks, Steve, for bringing it
up!  (of course no one's arguing about the color of PO4 supplemented water
when placed in a bucket yet, so it may not be quite up to the standards of
some past discussion ;-)