Soil pH

The pH of a soil is an expression of the concentration (or, more precisely, the activity) of hydrogen (H+) ions in the soil. The higher concentration of hydrogen ions, the more acid the soil. The hydrogen ions can be counterbalanced by ‘base’ ions such as calcium, magnesium and potassium; the higher the concentration of these ions, the more alkaline the soil is.

Generally speaking soils tend to become more acidic over time. This is due to rainfall (which is slightly acidic), microbes in the soil releasing hydrogen ions as they respire and crops taking up nutrients (in particular the base ions which counterbalance acidity) and then being harvested, so the nutrients aren’t returned to the soil. The extent to which a soil is affected by these changes depends on the type of soil. Soils which have a lot of clay or humus in them tend to have a higher ‘buffering capacity’, which means that they can balance out changes in pH by holding onto some ions and releasing others, eg buffering increased acidity by holding onto hydrogen ions and releasing calcium ions. This means that sandy or silty soils tend to be more susceptible to pH changes, particularly increasing acidity.

Understanding the acidity of your soil is important for several reasons:

• Different nutrients become unavailable to the plants or reach toxic levels at particular levels of pH. For example, very acid soil can cause phosphorous to be unavailable and aluminium to become toxic. Very alkaline soil can reduce the availablility of iron or cause toxic levels of boron.
• Soil organism activity (which is critical for breaking down organic matter and making nutrients available to plants) can be suppressed by extreme levels of pH. This is good if, for example, you have acidic soil because you should avoid clubroot (which perfers alkaline conditions) but not so good because you’ll also have far less earthworm activity to break down organic matter and aerate your soil.
• Plants have differing tolerances to acid/alkaline soils. Plants prefering more acidic (or ‘ericaceous’) soil include rhododendrons, ericas (heathers) and blueberries. Alkaline soils are preferred by plants such as celery, daffodils and lettuce.

pH is measured on a logarithmic scale (so one unit change represents a ten-fold change, two unit changes a hundred-fold change, and so on) from 0 to 14. Soil pH generally only ranges from 3 to 9 (though most are between 5.5 and 7.5). This is an illustration of the pH scale with some common substances indicated: