[APD] Substrate fertility & growth rates

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

Tom wrote:
> Adding more nutrients to the substrate will
> not force plants to grow faster.

Thomas, I think know why you said this because growth may be limited by CO2
or by lack of another nutrient. Also more than adequate growth rates can be
attained using only NO3 in the water and you desire a simple regime suitable
for newbies.

We should be cautious about saying that hydroponic regimes (excepting Fe)
are just as good as geoponic (substrate fertilization) at producing high
growth.

Recall from our recent messages, the C blassii grown in the deep soil
substrate was much larger (28") than the hydroponic ones (18") you
mentioned. Of course final size is not the same as growth rate but still
desirable. You didn't mention if 18" was the maximum size attained. Tank
size & competition are confounds too.

My understanding is that scientific research tends to support better growth
with N & P in the substrate. Here is what Dr Dave wrote to me in 1996:

  "     Preference for root uptake of P and N:

  "In a landmark paper published in Science 207: 987 to 989 (1980) two
Canadians by the name of Carrignan and Kalff clearly demonstrated that
rooted aquatic plants will absorb the majority of P from the sediment EVEN
UNDER HYPEREUTROPHIC CONDITIONS! This was the first paper which
unambiguously showed the preference of aquatic plants for sedimentary P.
Nitrogen studies are somewhat less clear, mainly because N is so difficult
to work with. Nichols and Keeney, Fresh.Biol.6:145 to 154 were perhaps the
first to indicate that rooted aquatic plants acquire the
majority of N from the sediment. After that was an excellent study by Barko
and Smart. They turned the question around somewhat and asked "which
nutrients can be left out of the water". Their results clearly showed that N
was not needed in the water column for optimal growth. I asked the same type
of question in a paper published in 1982 and came up with the same result
(Huebert and Gorham (Aquatic Bot. 16: 269 to 283). These experiments, and
several others, indicated that with a fertile substrate the only nutrients
required in the water column are Ca, Mg, K and of course CO2 (I must admit,
though, that the evidence for micronutrients is scant and it is in fact
likely that water column additions of micronutrients is a good strategy ...
though my swords and Sagittaria are completely indifferent to whether or not
I add micronutrients) Little work has been done since the mid eighties since
the dozen or so published studies were fairly clear in their results and
there is not much money for this type of research (its also difficult and
time consuming).

  "On the other side of the coin, there have been a plethora of studies
which indicate clearly that rooted aquatic plants will not grow optimally on
a sand or other infertile substrate no matter how richly you fertilize the
water column (perhaps the earliest is by Pond, 1905)"

Thomas, you've also mentioned good CEC as a desirable quality of Fluorite.
You must feel there is a benefit to having nutrients attached to those
binding sites. Note that the highest CEC is from organic material: peat
(100-150) or humus (200 cmol/Kg).

BTW, I never try to FORCE my plants to do anything. They are completely
indifferent to my begging, pleading or threats. I'm sure they do not
appreciate anthropomorphic comparisons. How uncivilized of them! I prefer to
think that they do like gentle encouragement & praise.   ;-)

Teban (too many Steves around here lately)


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