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Re: PO4 additions

On Sun, 20 Feb 2000, Karen Randall wrote:
> One more point about phosphate limitation in tanks that goes along with
> your comment about the E. beheri which outgrew the tank.  When phosphate
> deficiency becomes severe, there are a number of growth problems that
> you'll see.  But at the same time, by keeping the tank at least SLIGHTLY
> phosphate deficient you can control the growth of the plants without some
> of the deformities and discoloration seen when other nutrients are not in
> adequate supply.  This is one of the problems I have with using iron as the
> limiting nutrient for algae.  Plants don't look their best when iron
> limited.  If only _slightly_ phosphate limited, they just tend to be
> smaller, and less likely to flower, something we're not usually
> particularly worried about in the aquarium anyway.

I find it very easy to equate growth limitation by some nutrient
with deficiency in that nutrient.  Is that really true?

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

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.

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.  Within limits we *should* also be
able to add limiting nutrients (getting a corresponding increase in
growth) without necessarily revolutionizing conditions in the tank.

Roger Miller