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kitty litter substrate

A few days ago, Josh Liechty wrote:

> Another nutrient question: how long does kitty litter retain its
> nutritional value for plants? My 20 gallon aquarium has had kitty
> litter for about 6 months. Do those of you who use it find that you
> have to replace it after a period of time?

Kitty litter (as with other clay products) doesn't have much inherent
nutritional value.  It's value is in it's ability to capture and
concentrate nutrients out of the water (that's cation exchange capacity,
or CEC).

Some clay products - including some types of kitty litter - contain
plant-available calcium and magnesium carried over from its previous
environment.  Other clays -- notably the sodium bentonites that may be
used for clumping kitty litter -- may contain no nutrients at all.  Any
nutrients that the clay brings with it will supply the plants for a time.
Ideally, as the plants use those original nutrients they are replaced
through cation exchange by other nutrients (calcium, magnesium, ammonium
and potassium) that the clays capture and concentrate from the aquarium
water.  If that process is at work then the kitty litter will last

In order for the cation exchange process to work there must be some
movement of water through the clay.  Water movment allows the clays to
replace nutrients used by the plants with nutrients captured from the
water. The same is true of other substrate materials that provided cation
exchange capacity.  If there's no water movement through the substrate
then the nutrient content of the clay will eventually be depleted.  More
precisely, without water movement the plants will exchange hydrogen for
all of the available nutrients in the substrate and the substrate pH will
eventually drop to levels too low to support most forms of life.  That's a
bad thing.

It makes little difference how the water is circulated through the
substrate.  The plants themselves will induce some water movement.  It can
be provided mechanically by UGF or RUGF (though this would have to be
unusually slow to avoid suspending the clay) or maybe even thermally by
heating cables.

There is one thing that makes a big difference.  A solid bed of clay is
almost completely impermeable to water.  Water won't circulate through a
solid bed of clay no matter how much pressure (within reasonable limits)
you try to put behind it.  In fact aquatic plant roots may not even grow
into a solid bed of kitty litter or other clay any more than the roots of
garden plants will grow through a solid clay pan.

In order to allow circulation, the substrate needs to be built of a small
amount of kitty litter mixed with a larger amount of medium to coarse sand
or fine gravel.  Kitty litter probably should not exceed 10% of the mix by
volume. The sand or gravel allows water to move through the substrate and
the circulating water can reach and replenish the clays.  With more than
about 10% clay by volume the circulation may be substantially blocked by
the clay.

When Dan Q. originated the kitty litter substrate idea I thought his
intent was that a small amount of litter should be mixed (along with
Osmocote pellets) into the bottom layer a sand or gravel substrate.  I
don't have his original article anymore, so I can't check what he actually
wrote.  I don't know where the idea came from to build substrates with
solid beds of kitty litter. Several people have tried it; have any
experienced long term success with it?

Roger Miller