[APD] nutrients at start-up

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

"Why do some suggest reducing nutrient input during start-up?"

Difficult to answer since we don't know the entire protocol of the
alleged suggesters; whether nutrients are non-limiting with their
strategy when they are supplied. There is an hypothetical growth rate
limit for plants even when all nutrients, light are maximized and other
confounding factors, such as toxins (metal ions) and nutrient imbalances
are minimized.

If you are starting up, the smaller plant mass may not require as many
nutrients, in particular nitrogen so the supply of nutrients available
to bacteria and algae is greater. Yes, bacteria and algae do grow faster
when nutrients are more concentrated, particularly when N or P are
"excessive". It may be easier during start up to maintain a proper
balance by using less nutrients (not ceasing them entirely) but in
particular nitrogen and phosphorus.

If you are using an organic soil tank, then it may also be very useful
to perform more frequent water changes to reduce the amounts of organic
nutrients in the water since these contribute to algae growth more so
than elemental minerals. We have to be specific about which nutrients
we're talking about.

It is also common to see various kinds of bacteria, diatoms and green
algae having blooms during start up of an organic soil tank. Aside from
fertilizer considerations, its also useful to clean more frequently,
provide as many snails as possible and to use a temporary blackout to
knock the algae down and give snails a chance to catch up eating.
Daphnia are also good for green water blooms which sometimes occur with
soil that is giving off ammonia from decomposition and reduction of
nitrates in the substrate. For an organic soil tank, I think you should
reduce N & P but maintain all other nutrients at the steady state target
levels.

In an hydroponic growth model, you simply provide the right balance of
mineralized nutrients in the water and change it frequently to maintain
this level. If the plants are clean and there is no "start-up"
transition happening, then the system should already be at its dynamic
equilibrium. No need to alter the levels or ratios.

Perhaps the most interesting question, if I reframe it, is whether root
development is: will be promoted by a lower ratio of chelated Fe in the
water or by a lower ratio of P in the water? Perhaps. Root development
will also probably be stimulated by higher nutrient levels in the
substrate. A lot of things are going to affect the root to shoot ratio.
Is a higher root ratio in any way desirable? I should think it is
favoured by natural selection in oligotrophic conditions. As Tom points
out, aquariums are seldom oligotrophic. If you intend to never add
chelated Fe, then a higher root ratio would be beneficial but there must
be Fe and something like peat in the substrate to provide reduced Fe.
The same argument can be made for P.

Is it possible to get away without chelated Fe in the water? I think you
could have enough Fe in the substrate that it never ran short until the
roots so filled the soil that the redox potential wasn't sufficient to
reduce Fe. Fe oxidation zones are visible in the substrate near the
glass as rusty oxide deposits. It would take a long time for Fe to
become short however I think you could run out of P relatively sooner.
Is there a way to have a stable reservoir of P in the substrate? I'd
like to know that too!

BTW, my mother only wears her boots in the garden. ;-)

Steve P in Vancouver

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