[APD] pH in the filters & root zone

[Raj <ggrk at blr_vsnl.net.in>]

         In the thread "Excel and U.V. light" Greg Morin explained

>The method by which plants
>extract iron from EDTA is when the EDTA-iron is pulled down into the
>root zone where the pH is much lower and thus conditions are
>conducive to extraction of iron by the plants directly as it is
>released from the chelate under the acid conditions or the plants can
>also utilize phytosiderophores to extract the iron directly.

         I have noticed that when I unclog my choked (Eheim 2217)
canister filter by washing out *only* the first (lowest) layer sponge,
the pH rises in the tank. The pH returns back to normal after a week
or so.

         Would this be the same reason as what Greg explained? and
how does this happen I wonder. Or what causes the pH to be lower
(Acidic) in the root zone.

Raj 

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