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PO4 additions

Michael Rubin wrote:

>I like the idea of supplementing enough PO4 to feed the plants, but still
>using a small enough amount that the tanks runs out of PO4 before it runs
>out of anything else.  During a good week tank remains phosphate-limited
>even while adding phosphate on a regular basis!  I think Karen Randall's
>recent post indicating her excellent results using tapwater with measurable
>PO4 bear this out.

Yes, that's exactly how I think of it.  I still want PO4 to be the limiting
nutrient, but that doesn't mean that the plants can do well while being
_really_ starved for it.  I do agree with Tom Barr though, that I've never
personally seen an algae problem that I could attribute to a lack of
phosphate in the tank.  

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've come to realize, in my limited experience, that feeding the same
>nutrients in the substrate and in the water column have different effects on
>the tank as a whole.  Seasonal changes may come to bear as well (SF water
>has a much lower KH in the winter).  I've found that both ways in moderation
>work better for me than relying on only one form of supplementation.

There are several reasons for that.  Some nutrients HAVE to be available
through the water column, while others are best taken up by the roots.  It
also depends on the plants being grown.  Some are just better root feeders
than others, and vice versa.