[APD] Occlusion of N within clay

["Steve Pushak" <teban at powersonic_bc.ca>]

Greg,

Has SeaChem ever done any research into the viability of providing N in the
substrate by occluding it within clay pellets or clay balls? Are you able to
share any of that information with us on the APD?

It is my understanding from postings several years ago that denitrification,
the conversion of nitrogen compounds to N2 as gas, by bacteria requires that
other nutrients, notably carbohydrates, be accessible locally to the
denitrifying bacteria. See
<http://www.ozestuaries.org/indicators/Def_denitrification.html> which gives
a macro-chemical reaction for denitrification and
<http://webserver.cr.usgs.gov/nawqa/splt/journals/PFENNING1.html> a study
which examines the effects of temperature & organic carbon upon
denitrification.

If we put pellets of a commercial fertilizer inside clay balls and put these
balls into the substrate, I think that N diffusion through the clay will be
so slow as to be negligible. Likewise carbohydrates & other carbon sources
will not diffuse into the clay either. The clay must be of sufficient fine
texture & uniform consistency as is the case with pottery clay.

We could design an experiment to measure N loss from a clay ball. Is there a
reasonable experiment to measure the N content within a clay ball after it
has been submerged for 3 to 6 months? I'm thinking that we could just take
the clay ball and put it into a blender with a litre or so of distilled
water and then titrate the N in solution. I'm guessing that running the
blender for 10 minutes or so would be enough time to extract the N from the
clay into solution. As a control, we could perform a similar extraction upon
an identical clay fertilizer ball which has not been submerged. Goes without
saying that we weigh the samples at the start to control the amount of N
going into the clay ball. By knowing the starting N value & extracted value
in the control, we have a measure of the effectiveness of the extraction
process.

** Question: I don't know how to go about precisely measuring the N content
of a water sample; can someone suggest a practical at-home method? I suppose
I could get some very rough approximations by using nitrate & ammonia test
kits.

I've seen aquatic plants benefit from Osmocote (R) Controlled Release
Fertilizer 14-14-14 inside clay balls. The MATERIAL SAFETY DATA SHEET from
CAROLINA BIOLOGICAL suggests that it is comprised of ammonium nitrate,
calcium phosphate, potassium sulfate, ammonium phosphate, & calcium fluoride
and that it is coated with a resin made from vegetable oil reacted with a
cyclic diene.

>From what I can gather from the web, ammonia is not transported within the
vascular system of the plant but is converted to amides or ureides and then
reconverted to ammonia in the shoots & subsequently to amino acids. Nitrates
& nitrites can be transported from the roots to shoots where it is reduced
to ammonia via nitrogen reductase & nitrite reductase enzymes. See
<http://www.nrsl.umd.edu/courses/nrsc401/mineralassimilation.doc>

Ammonia as the sole N source causes acidification of the rhizosphere
therefore a mixture of ammonia & nitrate yields better results. See
<http://www.hort.purdue.edu/newcrop/ncnu02/v5-453.html>

It has been said that aquatic plants absorb N as ammonia in the substrate;
perhaps this is because ammonia predominates there, not because of a
biological imperative.

Please correct any inaccuracies I've made.

>From my experiences, it takes a while (weeks?) before plant roots can grow
root hairs into the clay ball to begin absorbing N from it even if the clay
ball is situated in contact with the primary root stem. Chemotrophism is the
property of plant roots which causes them to grow in the direction of
nutrients. If you uproot a plant grown upon a clay fertilizer ball for
several months, you will find a mass of roots around & penetrating the clay
ball. Therefore it would be wise to provide N hydroponically at the least
until the roots have penetrated.

If we assume that the majority of N from a clay ball is removed via the
vascular transpiration stream, will the plant utilize all of this N or will
some of it leak to the surroundings? Presumably there is leakage due to
normal tissue senescence. Presumably not all N is relocatable (mobile)
within the plant.

I doubt that even after several months, that the plant roots can entirely
drain a clay ball of N; other factors would limit growth & uptake; the plant
can only use so much N. Another experimental study could measure the amount
of N remaining in a clay ball after 3, 6 and 12 months.

All this work can be done without resorting to tissue analysis but I think
tissue analysis would be the best way to evaluate the effectiveness of such
a substrate N strategy. Certainly we can observe growth rates, leaf color &
size as well as the response of filamentous algae to a lowered N
concentration in solution.

Steve P

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