The test shown does not measure
pore water, only what leached into/out of the water column. Fresh Iron rich clay absorbs PO4, no surprise there.
Also, if the tap is 0.5ppm and you add plants and do 50% weekly water changes, then they have a lot of PO4, regardless if it's in the sediment or the water column. The plant leaves will get the PO4 before any
binding occurs in a planted tank, not a jar.
As far as algae and PO4, this is has long been shown not correlated in nature as well as aquariums:
http://fishweb.ifas.ufl.edu/Faculty%...macrophyte.pdf
I suppose 319 lakes is not enough data?
Also, sediment has a
limited capacity to absorb nutrients at which point is no longer becomes a sink, rather, a source and leaches them out. So this will only last so long before it no longer absorbs PO4. Given the amounts of PO4 I've added, the stuff is all filled up at the binding sites. I have no issues.
Less PO4 does not save anyone from more serious algae issues, this has been shown again and again for over a decade by countless hobbyists and simple test. Poor CO2? Too much light? Many have shown this causes many algae issues and poor plant growth.
I'd say that some plants have better adaptation to some nutrients than others at the root uptake, but I know of no plant that cannot take up nutrients from the leaves.
As far as when a problem does occur, I think the root of the issue is not PO4, this is a secondary issue, and one that does not limit algae.
Take a long hard look at the levels of PO4 required to show limitation in algae species that we might commonly see. They are beyond any test kit you own.
Call up the South Florida Water management district which deals with the Evergaldes water quality, ask them how much PO4 is required to limit periphyton growth and then plant growth.
I did.
I spoke to the lead researcher. She gave a nice presentation for the class.
20-50ppb for pants, less than 10ppb for algae/periphyton.
They do not even know how low it is, because they, nor any research labs has the ability to test at such low levels. A big problem for them.
If you add fish and their waste, then you have far more than this concentration of PO4.
Add any leaching out, or from the leaves of the plants(note; plants are sinks and sources of PO4, something the jar test does not consider) and you rapidly get well beyond limiting levels for algae growth.
regards,
Tom Barr