[APD] RE: peat moss / ph

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

Anna R. Dunster
> >Peat is very good at controlling the pH if that's what you want to
> >achieve.
> >
> >It will also reduce your hardness for several months as it
> soaks calcium
> >and magnesium out of the water, so its good to maintain regular water
> >changes and mineral dosing.
>
> Well that too is not a bad thing, my water is very hard, very alkaline
> and has high alkalinity ;)  But if I have to maintain regular water
> changes I should wait until I'm done with my coppersafe dose to start
> adjusting ph, right?  (probably would anyway as there is no point in
> stressing the fishwith additional changes)

Adjusting pH? You don't adjust pH using peat; see below.

> What I am really looking for here is not "how to change ph" because I
> have already decided peat moss is going to be the most feasible option
> for this tank. But I would like to know, how to most effectively use
> my peat moss.

I will defer explanation of acid/base buffering to those more qualified.
Please note that there are excellent explanations of this given by Roger
Miller, among others, in the APD archives.

Peat contains a fairly large amount of diverse organic acids. When I use
the cheapest, bulk peat available, Canadian sphagnum peat, it will bring
my water to a pH of 6.5 to 7. The pH will stay at that point until
enough of another acid or base is present to react chemically with the
organic acids in the water until a rapid change in pH occurs and then
you go to a new pH. The pH is not "adjusted" incrementally, its "locked"
to 6.5.

The pH buffers sold for aquariums differ in that they rely upon the
carbonate buffering system. I won't attempt to describe how this works
but you get a more continuous variation in pH with additions of CO2 or
carbonate.

Two things will happen when you add peat to alkaline water containing
dissolved calcium, magnesium and carbonate ions. 1) The carbonate buffer
is going to be affected by the organic acids. 2) calcium and magnesium
will be absorbed by the peat until it is saturated. These actions are
independent, so you can have peat saturated by calcium ions that still
has plenty of organic acid buffering left or vice versa.

It is difficult or impossible for me to quantify how much peat to use.
If you keep adding fresh peat and throwing out the old, you will remove
a majority of the calcium and magnesium required by your plants.

I suggest that you take about 1 cup of peat and mix it with 1 cup of
soil and put it into 1 gallon of water and measure the pH daily for a
couple of weeks. This will tell you a lot. You can also remove a precise
quantity of water after a period of time, say a week and boil it dry and
weigh the residue. Compare this with the residue from the same quantity
of tap water which has been boiled dry. This will tell you how much
calcium & magnesium is still left in your water. I suspect it will be
zero.

Using soft water, I've kept small aquariums with peat where the pH never
varied for over a year despite several water changes. At that time I was
not adding any minerals and was keeping killifish.

Peat has a CEC of 100 to 150 cmol/kg. That means that 1 kg of peat will
absorb about 0.15 to 0.10 moles of univalent ions. Calcium is divalent
so we can guess that 1 kg of peat might absorb about 0.05 moles of Ca
(but it could be more). That's 2 grams of Ca or 3.5 grams of CaCO3. CEC
is often measured independently for each type of ion species that it
absorbs.

If you are using fresh peat weekly or monthly, you will probably be
removing ALL of the calcium and magnesium from your tap water and your
plants must have that.

Background: the molecular weight of calcium is 40.08, carbon is 12.011,
oxygen is 15.9994. One mole of calcium weighs 40.08 grams. A cmol is
1/100 of a mole. Plug "define:mole" into Google for more information.

I apologize in advance since my chemistry is not strong nor do I have
the free time to do the research to properly answer your question.
Perhaps Roger might add more or he may be inclined to repeat the advice
given previously to use CO2. It might be useful to estimate how much
carbonate acid would be "used up" by a given quantity of peat. You'll
have to do the Internet research to discover the quantity of organic
acid in peat; I have no data at hand.

I hope the complexity of this explanation serves to illustrate why
carbonate (& CO2) are used in aquaria for pH buffers! ;-)

Steve P

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