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Re: PH <7.0 for plants?

>From: "Hopkins, Samuel" <Samuel.Hopkins at marconi_com>
>Subject: PH <7.0 for plants?
>Is it generally true that with plants you want the PH to be <7.0? And if so,

Yes it's true.  Mostly because some micro nutrients tend to precipitate out
of solution to a greater degree at pH values above 7.0.  This has nothing
to do with plant growth requirements but everything to do with chemistry.
It's sort of annoying to be adding micro nutrients to your aquarium over
and over and not be able to detect them with a test kit in proportion to
what you are adding.  Why?  B/c at pH 7.5 - 8.0 a significant portion of
the micros (read: Iron) have precipitated out of solution sticking to the
objects in your aquarium where they are unavailable to your plants.  These
precipitated micros do not go unnoticed however, and are consumed by brown
"algae" growing on the glass.  Sound familiar?

Be advised that w/o a carbon source, dropping your pH below 7.0 by itself
will do your plants no good.  Plants will grow only as steadily as their
least available nutrient. This is the principle I believe to be behind
those "Liquid Co2" products now out on the market.   My guess is they
contain an acid (probably Citric Acid) and a carbonate source.  The reason
we don't hear this approached discussed much is because acid dosing ruins
our ability to use those Co2/pH/kH  concentration charts we love so much.
It is a different approach to growing plants and is not well utilized or
understood in Aquarium plant circles.  It is however,  well known to the
hydroponics crowd where economies of scale make monitoring / controlling pH
and Carbonates via dosing pumps and TDS (Total Dissolved Salts ex;
Carbonates) cost effective.   The details are quite unimportant for our
purposes here.  However, when looking at this chart:
http://www.floridadriftwood.com/plantinfo/pH_level.htm it is easy to see
why hydroponic nutrient mixes are designed to be used at pHs like 5.5 - 6.5.

Does that mean you can't grow plants at pH 7.5?... 8.5?... 9.0?  Sure you
can.  If you have a fertile substrate.  Why?  'cause anareobic bio action
in the substrate will create acidic conditions that will aid in nutrient
uptake via the roots.  If under these conditions, you supply your plants
micros via their roots and you'er not just depending on nutrient dosing the
water column to supply your plants via leaves and stems.  If you are just
column dosing at very high pH values (in a new, plain gravel substrate) you
are wasting nutrients.

Aquarium Plants & Driftwood