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[APD] N transport from roots
As for N transport from roots to shoots: this is not a problem. Aquatic
plants do this easily.
You could argue that the plants require less energy if they get N at the
leaves and that's true. So what? We do not need HIGH growth rates in
aquaria; quite the opposite; we need moderate or low growth rates.
It is entirely possible to concentrate certain nutrients at the roots.
This reduces fluctuations in the supply of N and provides a steady long
There are two strategies for concentrating N (or other nutrients) at the
2) by occlusion
Occlusion means that the nutrient is encased in a dense medium that
inhibits diffusion. Clay is one such medium; polymer coating is another
method used for "slow-release" fertilizers such as Osmocote however the
effectiveness of a polymer coating is not very good underwater. It works
much better in your potted house plants. Jobe's Sticks do a similar
thing but I don't know what they are made of; it looks like some type of
cellulose composite but its probably a closely guarded trade secret.
The nutrients that can't be kept in the substrate are potassium,
magnesium and calcium. These have to be present in the water. They
should be present in an appropriate ratio however the tolerance is quite
wide. The reason for this is that an over abundance of one can be
antagonistic for uptake of the others. Calcium and magnesium are large
molecules and therefore difficult to transport. Terrestrial plants
normally have to get their Ca & Mg (and the majority of other nutrients)
from the soil but they have the benefit of a strong transpiration stream
fuelled by evaporation at the leaves.
When I talk about concentrating N at the roots, I don't mean that there
is no leakage. There is leakage. We can keep the concentration of N
about 10 or 100 or 1000 times as high in interstitial water than in the
overlying water depending upon the porosity of the substrate. Sand is
extremely porous. Only clay or a mixture of clay, silt and sand can be
considered non-porous. There has to be enough clay to fill-in the huge
spaces between the sand particles. [well huge is a relative term; think
of it from the perspective of a very tiny germ only a few microns in
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