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reducing phosphate leaching

> From: Luca Specchio <luckyluca at mclink_it>
> Subject: Re: Re: fertilizing both substrate and water column
> >Date: Mon, 10 Aug 1998 07:12:49 -0400
> >From: Neil Frank <nfrank at mindspring_com>
> >Subject: Re: fertilizing both substrate and water column
> >
> >>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. 
> Maybe if you have the 1st layer of substrate made of a media that has a
> very high CEC and that can be porous, the release process will slow down.

Phosphate is leached rapidly from decomposing organic material. If you
put fertilizer pellets in clay balls or use the Job's plant stix, the
diffusion rate is greatly reduced. I think using a small amount of
fertilizer (about 10 pellets) in a 1/2 tsp ball of clay might be a good
ratio of clay to fertilizer.

Organic matter such as humus has a fairly high CEC however I don't think
its enough to contain a large amount of nutrients. It may already be all
charged up and ready to release nutrients by exchanging calcium from
your water.

If you put a clay cap over your substrate, I suppose it might trap the
nutrients in there. I'm not sure that's a good plan however. For one
clay makes a heck of a mess if you disturb it. For another, oxygen
diffusion into the substrate is desirable too to prevent it going low
redox too rapidly. -balance-!!