Understanding Soil pH
© Frances Michaels
On very poor soils it may be difficult to grow anything, including even a successful green manure crop. In this
case the pH should be tested for excessive acidity or alkalinity, as this will interfere with uptake of nutrients
by plants. pH is a measure of the acidity and alkalinity of the soil using a scale from 1 to 14; where 7 is
neutral, less than 7 is acid and greater than 7 is alkaline.
Fresh, clean water is neutral with a pH of 7, vinegar or lemon juice is very acid with a pH of 2.6 and baking
soda is very alkaline with a pH of 8.5. It is important to remember that pH is a logarithmic scale, so the
difference between a pH of 7 and a pH of 6 is 10 times the acidity, between 7 and 5 is a 100 times the acidity
and between 7 and 4 is a 1000 times the acidity so it is obvious that this will have a major impact on the
ability of plants to grow.
pH is used as an indicator of the availability of other nutrients in the soil but only hydrogen ions are
Acid soils with a pH of less than 6 commonly have deficiencies in:
Acid soils with a pH of less than 4 commonly have toxic amounts of:
Alkaline soils with a pH of more than 7 the following nutrients may be unavailable:
Adjusting the pH will make these nutrients available to your plants. Organic matter will generally 'buffer'
plants against the impact of acidity so that a soil with a lower pH range will still successfully grow plants.
Plants vary in their desired pH range and this is to do with the pH of the soil type they evolved in. For
example, lavenders are native to the limestone soils of the Mediterranean and so prefer an alkaline soil.
If the soil is too acid, then agricultural lime (calcium carbonate) should be applied. The amount needed will
vary depending on the pH and the soil type. As a rough guide apply 120 g/m2
to a clay soil and 30
to a sandy soil. Test again in a few months and apply more if necessary. Agricultural lime is cheaper to
buy than dolomite (a mixture of calcium carbonate and magnesium carbonate). Dolomite is only a good idea if your
soil is deficient in magnesium. For example, many of the acid soils in SE QLD are already too high in magnesium,
adding more is a waste of money and can cause the ratio of calcium to magnesium to be out of balance.
Sulphates of iron and ammonium, elemental sulphur and organic matter are used to lower the pH (increase acidity)
of the soil, when necessary.
Gypsum (calcium sulphate) does not alter the pH of the soil but can improve aeration and reduce compaction
in a clay soil.
The texture of the soil e.g. clay or sand and the amount of organic matter present will affect the quantity of
material needed to alter the pH. Clay soils need a much greater amount of lime to shift the pH than sandy soils.
The addition of organic matter is always beneficial to the soil whether added as manure, compost or by green
manuring. Organic matter will generally 'buffer' plants against the impact of acidity so that a soil with a
lower pH range will still successfully grow plants.
Plants vary in their desired pH range and this is to with the pH of the soil type they evolved in. For example
lavenders are native to the limestone soils of the Mediterranean and so prefer an alkaline soil.
The following table sets out the amount of lime needed
to raise the pH of different
types of soils.
From RW Pearson and F Adams (eds) 'Soil Acidity and Liming':
Soil pH Test Kit
||pH 4.5 to 5.5
||pH 5.5 to 6.5
|Sand, loamy sand
pH test kits are easy to use and all gardeners should have one. This is the only reliable soil test you can do
More information on mulch
More information on organic soil improvement
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Cool season green manure seeds and kits