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Understanding Soil pH Frances Michaels
Raising pH   Lowering pH  

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 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
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.

Raising pH
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 g/m2 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.

Lowering pH
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 Texture pH 4.5 to 5.5 pH 5.5 to 6.5
Sand, loamy sand 85 g/m2 110 g/m2
Sandy loam 130 g/m2 195 g/m2
Loam 195 g/m2 240 g/m2
Silty loam 280 g/m2 320 g/m2
Clay loam 320 g/m2 410 g/m2

Soil pH Test Kit
pH test kits are easy to use and all gardeners should have one. This is the only reliable soil test you can do at home.

More information on mulch
More information on organic soil improvement
More information on cover crops and living mulch
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