I have been growing duckweeds in Clark solution (pH 6.5) and have noticed a 2 log pH shift down . I have checked the solution (blank) and there is no pH change in it. Does anyone know of an explanation of what they could be releasing in the water? I know that there are studies in the literature that indicate that the pH goes up and others indicate that the pH goes down, but I confess I am surprised by this value (from 6.5 to 4.5 in less than 7 days of cultivation)
Any plant that takes up more cations than anions excretes hydrogen ions into the rooting medium to maintain charge balance. Are you sure that there is no nitrification in the planed media because that too is acidifying?
In addition to Paul Milham 's reply, what other organisms are in the growth solution? Bacteria or phytoplankton could be growing in there living on the underside of the leaves or the roots/surfaces of the tanks etc.
How well buffered is your solution? If you haven't got a sufficient mass of buffer (MES or something along those lines) then all sorts of reactions could be affecting the pH.
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