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Re: KH2PO4

> What is the role of KH2PO4 in the fertiliser scheme?
> It is surely not part of the PMDD formula.
> Dr Athale, 
> Baroda, India

Some history:

Someone way back when was scratching his head because I had been bringing
loads of plants to the auction every month of all sorts. Going over to my
place and noticing my tap water was about 1.2ppm of PO4 and my iron levels
were 1.0ppm and there was not any algae made him rethink this notion of a
PO4 limited tank.
If PO4 caused algae, then where was Tom's algae?

Steve's tap water had 0.0ppm of PO4. So off to market and he decided on
KH2PO4. Very conservative at first, others went further. I was comfortable
at 1.0ppm and watched the PO4 drop down to .2ppm by weeks end where I did
another 60% weekly water change and fed the fish well.

I think one thing folks fail to see in the PMDD and it's theory, it says
limited but not absent. And it does work, but it was a lower light tank. At
higher lighting/CO2 enriched this PO4 is used more but can also be used at
lower lighting , just not as fast(so you don't add it nearly as much).

Each role of each nutrient was not as clear then either. Folks had been
blaming the PO4 when it was their CO2 or NO3 levels most often. Poor test
kits, assumptions on having to use using soft water, more algae eaters
available from the LFS etc.
Things are clearing up much more these days on plant health and nutrition.
Much of the focus has been on algae rather than plant growth. Universally,
folks profess that good healthy plant growth is the best cure for algae.
I certainly agree. Look into the maintenance levels required for algae. Then
consider a plant. 

Paul and Kevin's paper really help move things to this stage where just
about anyone can grow about any plant if they keep things within a nice
range of parameters without much algae. It certainly give a wonderful primer
into the role of nutrients in planted aquariums.
But limiting algae with nutrients can work but the plants will suffer more
than algae in the long term. I kept looking at the plants, not the algae.
Sure I had nice BBA and all the other algae at one time. So did folks with
0.0ppm of PO4. I "cure" it by upping my CO2 beyond the normal ranges to
20-30ppm and keeping it there consistently. Adding K+ and NO3 to good levels
filled in the rest. Traces were also added based on eyeballing a volume(in
mls) to tank size rather than direct testing. I noticed I had much higher
traces. Claus from Tropica confirmed that folks where adding too little
traces to their tank when he came here a few years back.
But you can also limit a tank using iron, PO4 perhaps others. But rather
than _limiting_ something for prevention of algae, I just gave the plants
what they needed to grow fast and healthy. I think the notion of limiting
anything to beat algae is going to be rough go for the plants also.
Algae can live at extremely low levels of PO4, NO3 etc.

This becomes much more apparent when you use high lighting. PO4, Fe cannot
be supplied via the gravel when this happens and this slows the plant growth
down. You can get away with this(limiting) at lower lighting but
interestingly, the levels can be also be nice and high at low lighting
also(non limiting) and you will get even better growth and no algae.
At 5-10ppm NO3, the algae are not limited in NO3, nor at 1ppm of PO4 etc.
All the nutrients are there for the plants AND the algae but the algae don't
grow, the plants do.
Tom Barr