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Clay balls or pellets in an all gravel substrate

Recently there has been some discussion by Steve Pushak and George
Booth on the subject of the use of clay balls and or pellets.   One of
the questions brought up was about the use of clay balls or pellets in
all gravel substrates.   In our experience we've found that clay pellets
composed of one or two parts clay to one part pmdd dry mix works
extremely well as a general fertilizer and controled substrate
additive.  In our experience we've found that it works better than
layered substrates of clay, laterite etc.  We think that the reason may
be that it offers better control of conditions than other approaches. 
We never use anything other than gravel anymore, and we have fantastic
success.  

A COMMENT ABOUT NATURAL PROCESSES:  The natural habitats of most
tropical aquatic plants differs greatly from that found in the
aquarium.  Most tropical aquatic plant habitats consist of slow moving
streamlets or contained ponds.  Water flows slowly through these
habitats in two main ways, through stream flow, and by percolation.  
The effects of stream flow in the aquarium can be accomplished by
periodic or continual water changing.  The effects of natural 
percolation however are extremely hard to achieve.   When water
naturally percolates through stream or pond bottoms the concentration of
nutrients increases in the bottom soils and decreases in the free water
around the plants.  Rooted plants thrive while primitive plants (algae)
are deprived and are thereby controlled.   In the aquarium on the
otherhand quite and unnatural situation is set up.  The bottom and side
of the aquarium are impermeable.  Nutrients do not move naturally from
the free water into the substrate, but rather move un-naturally from the
substrate into the free water.  Only with the greatest of good fortune
and or skill can one set up a layered high nutrient substrate without
having some significant control problems.  Some folks even install
heaters to get the "action" really cooking.  We never heat our gravel
substrate.  
	We believe that the most prudent approach is to add clay/fertilizer
balls or pellets sparingly underneath individual plants, so that there
is never enough of this material that it can get "out of control".  In
nature water percolation controls concentrations, in your aquarium you
should set up an environment wherein you have tight control of the
concentrations.

  Interestingly, we've found that the small controlled amounts of 
fertilizers we add via clay pellets do not for the most part migrate
into the water column.  We have performed an experiment wherein we added
an amount of fertilizer in pellet form to the substrate which if added
directly to the free water would have registered significantly on our
test kits.  But yet a day later, 2,3,4,5,7, still no reading.  This is
contrary to prevailing wisdom which says that substrate fertilizers in
gravel migrate quickly into the free water.  In another experiment, 
fertilizer/clay pellets were placed into test tubes with a 1" layer of
gravel below and 1" layer above.  Within a day the clay pellets had
disolved and settled through the gravel to the bottom of each tube, no
matter how fine the gravel was.  We've observed the same thing in
planted tanks.   This action  sets up a very fine layer of fertilized
clay at the bottom of the tank which seems to be a more controllable
situation than a massive laterite or clay layer.   
	That's our view and experiences, but your water conditions etc may be
so different that our results may not work for you.  That's one of the
reasons why this is such a great hobby, you'll just have to rise to the
challenge to find out what works for you.               

-- 
Bill Terburg
Plantguild
Email: terburg at familychest_com
Web:   http://www.familychest.com/plantguild/