UNDERSTANDING SOIL pH

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, 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 ten 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 actually measured.

Acid soils with a pH of less than 6 commonly have deficiencies in:

• Calcium
• Magnesium
• Phosphorus
• Potassium
• Molybdenum

Acid soils with a pH of less than 4 commonly have toxic amounts of:

• Aluminium
• Manganese

alkaline soils with a pH of more than 7 the following nutrients may be unavailable:

• Iron
• Manganese
• Zinc
• Copper
• Boron

The addition of agricultural lime (calcium carbonate) or dolomite (magnesium carbonate & calcium carbonate) will increase pH (decrease acidity) of the soil. Agricultural lime is cheaper to buy than dolomite. Dolomite is only a good idea if your soil is deficient in magnesium. 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.

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 eg 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 & F Adams (eds) ‘Soil Acidity & Liming’):

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