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Re: Clay balls or pellets in an all gravel substrate

For some reason I thought that the Plant Guild was an organization of
plant keepers.  I think that was my assumption, not something I was
intentionally mislead into believing.  For those who have not checked out
the Plantguild web site;  Plantguild manufactures and sells equipment for
making and applying fertilized clay pellets.

On Sat, 23 Jan 1999, Bill Terburg wrote:

> 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.  

I found this use of the royal "we" a little unusual until I realized that
Mr. Terburg was speaking on behalf of a commerical venture, not for a
plant club.

> 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.

Don't forget swamps and marshes.

> 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.

In many water bodies wind-induced currents are a very noticable (even
dominant) cause of water movement.  The wind induced currents move water
about within the water body without causing a periodic or continual water

> 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 most natural conditions the percolation of water through the bottom of
a water body is extremely small when compared to other causes of water
movement. There are exceptions of course, such as fens and spring-fed
ponds sheltered from the wind.  The volume of water percolating through
the bottom is so small that it usually is completely ignored in
engineering work; in most environmental studies it gets no more than a
passing nod.

> 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.

Under most natural conditions the net flow of water is from the pond
bottom or stream channel *into* the water, not the other way around.  
Exceptions to this are mostly in arid zones and temporarily, as a result
of short term increases in water level in the pond or stream.

Plants through transpiration might induce movement of water down into the
substrate around their roots, but even in riparian zones and zones of
emergent vegetation, the transpiration requirement may be satisfied
entirely from the ground water supply without inducing any water movement
from the water body to the substrate.
> 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.

That's a reasoned approach.  Many people on this list use frequent small
doses of fertilizer and test their water to achieve the same tight control
on nutrient 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.

Clay balls are essentially impermeable to water movement.  That
impermeability prevents nutrients from leaving the clay ball with moving
water, so once you've put the nutrients into a clay ball it can only leave
through diffusion, which is a slow process.  Further, clays are composed
mostly of small, negatively charged particles.  The electrical attraction
between clay particles and positively charged nutrients (which is most of
them) may further impede nutrient "leakage" from the clay balls.

Nutrients in a clay ball are not readily available, so it isn't surprising
at all (to me) that you wouldn't find them appearing in the water column
over a period of a few days.  

> 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.

The observation of clays "dissolving" (I assume you mean that the pellets
broke up rather than actually dissolved - clays are not soluble in water)
is a little surprising.  Pure clays form cohesive aggregates - they stick
together.  If they don't stick together then most of the nutrients you've
put into them (excepting the part that is actually exchanged onto
the clay particles) should be free to roam about in the water.

> 	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.               

I don't dispute that fertilized pellets may provide a way to get nutrients
into the plants' root zone.  I obviously do dispute some of the reasoning
behind the method.

I guess I'd feel better about this post if it came from a satisfied user
of the method rather than from the manufacturer of equipment and supplies
used in the method.

Roger Miller