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We often hear gardening gurus talking about the
"pH" of soils, and it can be very confusing.
Basically the pH scale measures how acid or how
alkaline soils are, or (technically) what is the percentage
of positively charged Hydrogen ions in the soil.
The pH scale measures from 0.0 (most acidic) to
14.0 (most alkaline). Distilled water has a pH of 7.0.
Not too many plants could survive in soils at either
of the extreme ends of the scale, and most plants are
happy in soils measuring about 6.0 to 7.0, or just below
We wouldn't have to worry about the pH, except
that some plants thrive in more acid soils, with a low
pH, and some flourish in more alkaline ones, or slightly
higher on the scale. The reason for this is that the
essential elements in the soil such as Nitrogen,
Phosphorus and Potassium etc, become more or less
available for plants to absorb, at different pH levels,
with the best availability of elements at pH 6.5 (Yates
Garden Guide, 1999).
Some of our most beloved plants in the cool climate
garden, thrive at the slightly lower or more acid end of
the scale - about 5.5-6.0.
You see them referred to as "acid-lovers" (always
sounds a bit suss and 1960s!) These include Camellias,
Magnolias, Rhododendrons, Azaleas and Daphne, as
well as Australian natives such as Callistemon, Grevillea
Most plants of the cool climate productive garden -
herbs and vegetables, on the other hand, like the pH to
be in the 6.0- 7.0 range.
In the Southern Highlands, our soils mostly have a
pH of 5.0 to 6.5, but since you don't always know the
history of the soil treatment in your garden, it is a good
idea to test the soil in various parts of it.
Any store that sells garden supplies will have pH
kits and they are easy to use, when you read and follow
If the test indicates a very low pH you can increase it
by adding dolomite lime to the soil at the rate shown
on the lime packet.
To lower the pH (make it
more acid, or more suitable for
Camellias etc.) add aluminium sulphate,
or iron sulphate. These will also make
Hydrangeas more blue.
In any event, you can't go
wrong adding well rotted cow
or sheep manure to the garden, to
improve the soil texture.
Even horse manure is good, but first put it
in the compost for a few months to get rid of the
grass and weed seeds.
This article was written by Margaret Stuart of the
Highlands Garden Society Bowral
Inc. Members and non
members are welcome at
the monthly general
meeting on February
17 at 7.30pm in
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