Re: fertilizing both substrate and water column

[Neil Frank <nfrank at mindspring_com>]

>From: Steve Pushak <teban at powersonic_bc.ca>
. 
>
>For our purposes it makes little difference to most plants if you supply
>N or P in the substrate or in the water column. Those nutrients quickly
>move into the water column under most circumstances. 

It is better to have P in the substrate. This keeps it away from algae
which can only get it from water column. The plant sticks (like Jobes) or
ozmocote or Delaware Aquatics plant tabs all contain water soluble
phosphates. (Is DAI still in business?) There is also the fern Jobes with
less P, but it is really high in N. I used it once and it may have put it
too close to the roots cause my plants yellowed which is a symptom of too
much N.
 Even when these terrestrial fertilizers are inserted deeply into the
substrate, they PROBABLY dissolve quickly and diffuse up into the water
column. I say probably, cause I have never seen any confirmation of this
theory with a phosphate water test. This would be easy to do with a series
of replicate gallon jars that each have 3 inches of different substrates
(sand, gravel, soil plus sand, laterite plus gravel, etc.) Bury a known
amount of fertilizer, calculate the potential P concentration (mg/L). The
concentration will be high enough to avoid test kit sensitivity. Then make
the test every few hours initially and then daily thereafter. Soil or
laterite with its iron should have the best chance of binding to the
phosphate and keeping it out of the water. But maybe Steve is right and it
all ends up in the water. The test can also be made with some floating
plants and rooted plants which will remove and store the phosphates. This
is the same "trick" used by water treatment plants and of course is what
happens in a well established plant tank.

I would also like to see someone come up with a small NPK plant tab (or
stick) that has a small amount of relatively insoluble phosphate. There is
probably one made for ponds that can be repackaged for aquarium plants.

Neil Frank
Aquatic Gardeners Association

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