Concentration range for K

I've been using the Conlin-Sears PMDD
for awhile now and have had great results!
I monitor P, Fe, and NO3, but have not found
a good means for measuring K.

Still, knowing how much PMDD I add, I can
get a feel for the upper limit on K
concentration through calculation (that is,
what K concentration would be reached assuming
no absorbtion by plants or anything else). Might
anyone know what range of K concentration is
desirable for aquatic plants? Do they
tend to use K in some ratio to a
measurable quanity like NO3, Fe, or P so I
could take a stab at depletion rates?

Thanks in advance for any help,
Tim - Pittsburgh