How to measure and adjust your soil's pH levels

Different plants have different soil requirements. One way to see what your soil may be lacking is to do a pH test, so here's what you need to know.

• Acidity and alkalinity are measured on the pH scale, which runs from 0 (pure acid) to 14 (pure lye)
• From the neutral point, 7, the numbers increase or decrease in geometric progression: thus, pH 5 is 10 times more acid than pH 6; pH 4, 100 times more acid; and so forth
• Slightly acid soil — about pH 6.5 — is best for most plants, but there are exceptions. Rhododendrons, for example, require pH 4–5.5 in order to thrive. Cabbage does best in slightly alkaline soil, about pH 7.5, partly because the clubroot fungus that can afflict it thrives in acidity
• Few plants will survive in soil more acid than pH 4 or more alkaline than pH 8

• If you do want to increase the alkalinity of the soil, use finely ground limestone, which is available from most garden-supply centres
• In fall dig the soil to a depth of 20 to 30 centimetres (8 to 12 inches)
• Spread the lime evenly over the surface, and rake it in
• The rate of application depends on the texture of the soil. In sandy loam 25 kilograms  per 100 square metres (55 pounds per 1,000 square feet) will raise pH a point; in medium loam use 30 kilograms (65 pounds). In heavy soil 35 kilograms (77 pounds) of limestone is required for the same pH change
• These quantities are to change the soil's pH from 5.5 to 6.5. The more acid the soil, the more lime is needed to raise its pH by one point. Increase quantities by about one-fifth if the pH is lower than 5.5

• To increase the acidity of the soil drastically, use elemental sulphur, which over a few months will turn to sulphuric acid in the soil
• Like limestone, the amount to use depends on the soil texture: in sand four kilograms per 100 square metres will lower pH by about a point; in heavy loam 12 kilograms (26 pounds) is needed
• For the most effective and long-lasting treatment, work the sulphur in to a depth of about 30 centimetres (12 inches), along with large amounts of leaf mould
• Leaf mould can also be acidic, depending on the trees it is made from (oak especially), and incorporating pine needles into the soil will also reduce the alkalinity
• Both work more slowly than sulphur but have the added advantage of improving the tilth of too light or too heavy soil

Whether you are raising or lowering pH, there is always the danger of overdoing it. Use only as much corrective material as you are sure you need based on reliable soil tests.