Re: PMDD's and nitrate
> From: psears at NRCan_gc.ca (Paul Sears)
> > From: Stephen.Pushak at saudan_HAC.COM
> > I have a week old _experimental_ soil substrate which I have recently
> > set up. I have used PMDD at a reduced dosage (but not reduced
> > enough) for the last week. Recently I've been measuring various
> > parameters and find that I have approximately 100ppm nitrate in
> > this tank.
> How long has this tank been going? The implication of what is
> written above seems to be that it has only been set up for a week.
> If so, the contribution of the PMDD's to the nitrate there now is no
> more than a few ppm.
that's true. The total dosage in that period of time would have
been about 40 ml standard PMDD dosage. I don't think the nitrate
all came from the PMDD but I wasn't sure how much would accumulate
in that short period of time. I'll test again tonight but I won't
be surprised to find the nitrate concentration high again. This is
not entirely unexpected since it is a pretty fertile substrate.
I had expected a higher phosphate concentration but that is only
at about .5 ppm. I didn't mean to suggest that PMDD was responsible
for the high concentration, only that it was inappropriate for me
to be using it at this stage. I do suspect I might need to continue
Fe dosing so I'll be making further measurements and adjustments.
> > o monitoring NO3 regularly is critical for proper use of PMDD!!
> I think we pointed this out in the original posting on the
> subject. Monitoring nitrate concentrations isn't a bad idea in _any_
> tank, and will be more important still in a planted tank, where
> nitrogen _must_ be available for the plants, yet accumulation is
> to be avoided.
Yes. I thought we ought to stress the importance of that particularly
for those considering various soil substrates.
> I'm not wildly enthusiastic about the "mixed" approaches to
> plant growing. If you have a fertile substrate, then why add things
> to the water column? I use the water column approach mainly because it
> is very easy to adjust, and I don't have to tear the tank down when
> the substrate wears out.
I wasn't sure how much Fe would be available to begin with. In some
cases a fertile substrate will not supply sufficient Fe. This substrate
is designed to determine if we can get sufficient Fe from the
substrate alone. The problem is in the startup period because Fe
will not become available until the redox in the Fe layer becomes
low enough and the bacterial cultures become established. This is
also an important time because it is the break-in time for the
establishment of the plants to get the head start on algae.
Another reason one _might_ want to do additions in the water column
is that Diana Walstad mentioned that plants have a preference for
absorbing Ca from the water column. One might want to add CaCO3
regularly. I don't know how well the average plant can absorb
Ca ions from the substrate. They have the highest affinity for
CEC sites and Diana mentioned that some plants are entirely
unable to absorb Ca from the substrate.
I have extremely soft water and to see if plants can use substrate
Ca I have added dolomite lime in a small amount to certain parts
of the substrate. I stress that at this time, this is experimental
and I do not advocate it (yet). Certainly, the lime is going to
have two effects:
1) The Ca & Mg ions will displace NO3 which may occupy CEC sites
This will liberate nutrients to the water column.
2) It will mediate the pH and encourage bacterial activity which
will stimulate decay and liberate even more NO3
The extra bacterial activity will increase BOD which will reduce
the redox which might have several results:
- increased Fe availability (possibly to the point of toxicity)
- increased ammonia in the substrate (could be good or bad)
- possibility of denitrification (conversion of ammonia to
N2 & other products)
- and if low enough, the production of methane and H2S
I'm not describing the proportions at this time because I don't
want anybody copying this yet. At this time, no fish are in
this tank so I can do water changes at will. I haven't measured
ammonia but it's possible that more ammonia will be generated
in the substrate than the plants can use. That may take a
few days or weeks as the substrate modifies with submergence.
The amount of dolomite lime to be added is going to be an extremely
critical factor to determine.
*** I really wish I had a redox meter!! ***
> > o in closed systems where water changes are minimal, we need to
> > be cautious about nutrient additions. These can probably be
> > accomplished (with possible exception of Ca) by regular feedings
> > of a balanced fish food diet (or other organic materials in
> > fishless tanks)
> "Balanced" for the fish may well not be "balanced" for the
> plants. Do water changes, throw out plant material as it grows; the
> faster the turnover of water and plants through the tank, the less likely
> is the buildup of anything.
This approach does not seem to be very popular at this time or
even well known. I know Diana Walstad uses this approach. I mentioned
it for completeness. I want to bring out the point that there are
at least three fundamental approaches and that mixing things
together without proper consideration can have undesirable
results. I think lots of folks are thinking about using soil
and PMDD since they are both popular but we need to stress
the appropriate cautions.
> > If you use small containers
> > for your Crypts or other plants, you can experiment with
> > much more freedom and I would encourage people to use this
> > approach and share their results (good and bad).
> Is that what you were doing in the tank mentioned at the beginning?
Not in this tank. It is a 50 gal tank with a three layer clay,
soil+vermiculite and gravel substrate. I have used the small
containers for some experiments with Crypts, A mad and Cabomba.
Incidentally, this is the first time I've had Cabomba grow
successfully and rapidly. It's in a soil/silica sand container
with enriched clay balls that I described previously. The
Crypts are in a clay/soil/silica sand container with the A. mad.
The crypts are growing well but the lace plant is not doing as
wonderfully as could be hoped. It has responded (I think) to
additions of CaCO3 in the water. This is also indirect evidence
that the important factor for A mad may actually be cooler
Sometimes you need to test things on a larger scale than just
in little containers. What works well on a small scale may
not on a large scale because you don't have the advantage
of the larger volume of tank water to absorb the effects. :-)
It also makes me wonder what would be happening in my tank
right now if I had followed the popular practice of innoculating
a new tank with water or gravel from an old tank. Probably
very green water and an outbreak of a variety of filamentous
Steve in wet & chilly Vancouver BC