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re: Solid fertilizer and 100 mg/l of NO3
Ordinary fertilizers are quite soluble. Gravel, sand and other
relatively coarse materials have little ability to prevent nitrate and
other nutrients from diffusing out of the substrate.
I think you are right to use a much smaller amount of fertilizer. You
might try just a few pellets mixed into clay balls and allowed to dry.
The clay forms a very dense and impermeable matrix as it dries. When you
put one of these dried balls into the substrate, it takes quite a long
time for the water to penetrate and the nutrient diffusion is very,
very, very much slower in the tiny spaces between the microscopic clay
particles (compared to gravel) which are fractions of microns in size.
The extremely fine pottery clays may be superior to the clay sand
mixtures you might find outside. The other benefit of using a clay ball
is that the clay itself is able to adsorb nutrient ions onto it's
extremely large surface area. We may also over estimate the ability of a
particular material to adsorb a relatively large amount of ions.
Can anyone tell us how many milligrams of NH4+ one kilogram of clay with
a CEC of 100 centimoles of exchangeable charge positions per kilogram
could adsorb if it were somehow able to completely occupy all of it's
CEC capacity with the ammonium?
Note: I think that the majority of cation exchange sites get occupied by
calcium ions! reason: 1) we need fairly high Ca concentrations (perhaps
to compensate for this) 2) the Ca ion binds more readily to CE sites.
Is anyone interested in repeating the phosphate diffusion experiment
done recently with a Jobe's stick but this time using a clay ball? I'd
be very interested to see if the experimental result matches the theory!
Aquatic Gardeners Association member