Iron debt

I have some Ferrous sulfate tablets I got at a health food store. 
They have 65mg of Fe++ per tab.  My tank is 50gal=200l=200kg, so one
tab should raise the Fe by about 325ppb.  I measured my iron as
negligible so I added half a tab and measured after 12 hours.  Still
negligible.  Added another half, wait, still neg.   Another half,
wait, sn.  Added a dose of micronutrient mix (incl. iron), wait, sn. 
Another half, wait, sn.  FIVE tabs and a big mn mix dose later, the
iron starts to show.  My kit will resolve down to around 20ppb, so I
know it was really low.  I also tested some high Fe water for
calibration purposes.

What it looks like to me is that the plants had a big Fe debt and as
I added Fe, they sucked it up (at least for a while).  Does anyone
have any measurements or experiments or experience to confirm that
such a thing is possible?    Dave

Dave Gomberg, Experimenta      San Francisco CA USA   gomberg at wcf_com