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Terminology, Nutrition and Substrate Longevity

Dr. Dave wrote:

>Growth is a technical term which refers to the rate at which plants 
>increase their biomass. It is generally reported as grams produced per 
>gram extant per day. 


>With this as a benchmark, it is clear that NO ONE on this list has plants 
>which exhibit 'good growth'. What successful aquarists have are stable 
>tanks with a high 'standing crop' of healthy populations of plants. Of 
>course, few would want to have to deal with optimal growth rates!

Thanks for a very good post, Dave!



Subject: Substrate longevity

Steve Pushak wrote:

>Karen mentioned having very old tanks that became rootbound to the point
>that they were difficult to manage.
>I believe that I read somewhere, possibly when communicating with Dave
>H, that the root to shoot ratio in plants is often determined by the
>availability of nutrients. A plant which is low in nutrients will divert
>more of its available energy into growing roots whereas a plant with
>sufficient nutrients will grow large leaves and stems. This makes sense
>because there's no gain in growing more photosynthesis surface if the
>plant can't access sufficient nutrients.
>This suggests that a root bound tank might be one which has been low on
>nutrients for a long time. 

I suppose that's a possible scenario.  In the case of my tanks, I am fairly
certain that it was more a case of too many too big plants in too small a
container.  Foliar/stem growth was still excellent (in aquarium terms ;-).
In fact, when I replanted the tank, I permanently removed about 60% of the
plant mass, and the 40% that I put back filled the tank enough that it
looked like a mature planting.  Steve, you actually saw before and after
photos of this tank during my talk.  Do you remember the one where the
Lilaeopsis was not yet standing up straight?  That was the day _after_
planting.  The tank looks pretty well "filled" with 40% of the original
growth.  And we're talking leaves and stems here, not roots.  

>If left on its own for long enough, a planted
>aquarium will eventually fill up with plants and as the substrate
>fertility is reduced, more root growth will be stimulated. 

I think another factor is that a root bound substrate is too aerobic for
good nutrient availability, whether the nutrients are present or not.

>I think the
>best strategy would be to periodically uproot several of the plants and
>sell them off. You certainly need to do this with several varieties of
>Crypts or they'll take over.

I think that's what I suggested.  It certainly isn't a good idea to let
things get so dense.  But inertia is hard to overcome!<g>

>Another point brought up by Bob Dixon about substrate longevity was
>about the need to add substrate fertilizer to replenish nutrients which
>are eventually used up. This is a good point but I think by substrate
>longevity, we really mean the useful life of the substrate. In this case
>we are perhaps extending the useful life of the substrate. 

Agreed.  I think that what George is saying, and I find no reason to refute
it, is that the cables make it unnecessary to manually supplement the
substrate.  I think either is an acceptable and workable approach.  You
have mentioned supplementing even your soil substrates with fertilized clay
balls.  I see no problem with this.

>Consider an analogy of houseplants which become rootbound. They do need
>to be repotted periodically. I think its often suggested that the roots
>themselves should be trimmed when repotting, "to encourage new root

What happens with a pot bound house plant is that the roots tend to keep
circling in their tight formation unless you make a few cuts to loosen
things up.  

>I don't know if there is validity to that but the idea of
>pruning roots in the aquarium has been mentioned once or twice and
>perhaps we could discuss that idea?

I think it is useful to trim the roots of aquatic plants because it is
difficult to replant a plant with very long roots without damaging them.  

Karen Randall
Aquatic Gardeners Association