[APD] Re: N in the roots

[Thomas Barr <tcbiii at earthlink_net>]

"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."

So with that in mind, add less light.
It makes far more sense than any other thing since it drives the CO2 uptake, thus drives the N uptake.

Limiting something with nutrients and placement seems a tad boody backwards versus using light which is the first and primary driver of photosynthesis.

"It is entirely possible to concentrate certain nutrients at the roots.
This reduces fluctuations in the supply of N and provides a steady long
term supply."

It's easier to use less light to control growth rates.
That provides a slower consistent supply, the substrate wears out and you have no way of telling when that is.
You can test and know the level of a water column fert.
It's also not as messy, Riccia , Ferns etc also do not have any access to substrate ferts.

"There are two strategies for concentrating N (or other nutrients) at the
roots:
1) chemically
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."

Not really.

"When I talk about concentrating N at the roots, I don't mean that there
is no leakage. There is leakage."

So why bother?

"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
size]

Steve"

So how do you consistently make something that has a known flux out of this medium?
Sounds like a hassle that has no benefits other than a lazy dosing routine vs water column.

If you add macro's to a cat litter with some moisture, add peat, cook it 1/2 way, then you might get something that might last a few weeks with a CO2 enriched tank.

Beyond that, either adding more(how does one do this evenly?) or dosing the water column will be needed.
So while initially the substrate dosing might seem easier, longer term issues come into play.
My thread about dosing in the start up phase seems to make the case to dose the water column vs substrates.

Fungal associations, even cutting the roots off show that plants(growth rates) are not effected when there is ample nutrients in the water column.

While all this is one notion/issue etc, I've entertained the notion of a time released 6-12 month supply of NO3/PO4.
But this needs to be a consistent release at a known and given rate or something only roots can tap into.

My main arguement for substrate macro dosing:
The roots are already there, adding nutrients to the waster column requires them to take them in from the shoots/leaves and transport them to the roots.
Both N and P are mobile and this is a reverse flow(vs the typical model) but plants can and ado do this very often in various stages.

But will having NO3 in the substrate hurt?
No, it's not abbad backup in case you space out a dose in the water column etc but it does take a few days to for a plant to adapt to a particular type and source of nutrient.
CO2 to HCO3 takes about a week.
Root to foliar takes perhaps a day or two.
Depends on the plant, health etc.


Regards, 
Tom Barr





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