Measuring and Interpreting Soil pH

Measuring and Interpreting Soil pH | pH and Nutrients | Acidification Animation

The chemistry of soil pH

pH is a measure of the activity of the hydrogen ion (H⁺) in the soil solution. If the concentration of H⁺ is high, the medium is said to be acid. If it is low, it is said to be alkaline.

pH is defined on a logarithmic scale, which means that a 1 unit change in pH is in effect a ten-fold change in acidity. In mathematical terms pH = log¹⁰ 1/[H⁺]

What ranges of pH occur in soil?

While pH is measured on a scale of 1 to 14, most agricultural soils are found between the range 4 to 10 (when measured in water). For practical purposes, soil is neutral when pH is between 6 to 8, depending on plant requirements, and it is acidic when pH is less than 6 and alkaline when it is greater than 8.

What ranges of pH occur in biology?

Acid fluids include vinegar (pH 3), gastric fluids (pH 1.2), lemon juice (pH 2.2), rain water in equilibrium with atmospheric CO₂ (pH 5.6), and even egg yolks (pH 5.6).

Examples of alkaline fluids include seawater (pH 8), blood (pH 7.4), cleaning ammonia (pH 11.5) and baking soda (pH 8.2).

Why is pH important?

The activity of hydrogen ions has a major impact on all chemical reactions occurring in aqueous mediums. Moist soil is a full of chemical reactions. pH affects the availability of many nutrients, symbiotic Rhizobia, and soil biology.

Many plants will tolerate pHs between 5.5 to 8.5. However, the tolerance to extremes in pH varies between plant species and within species. Some plant species have quite different preferred pH ranges (e.g. lucerne 6.0 - 8.5, celery 6.0 - 7.0, potatoes 5.0 - 6.0). Therefore, consideration of the need for soil amelioration will depend on individual circumstances.

Measuring pH

pH is determined by measuring the hydrogen ion activity in an aqueous solution. A glass electrode, calibrated against a pH standard is used to do this.

Measuring pH in soil

To measure pH in a laboratory
A sub-sample of soil is mixed with water or CaCl₂ at a ratio of 1 part soil to 5 parts liquid and the pH of the suspension is measured after 1 hours shaking. Most laboratories are equipped to automatically measure electrical conductivity and pH in 1 part soil to 5 parts 0.01 M CaCl₂ at the same time.

The 0.01 M CaCl₂ method has been used in recent times: it is a more robust method of measuring pH than measuring in water as it is less affected by soil salts. It is important that the method used to measure pH is given when interpreting soil pH (e.g. pHw). All pH values presented on this website have been measured in water.

To measure soil pH in the field

A field pH test kit with a colorimetric indicator can be used on site, or the laboratory method can be roughly imitated by shaking 1 part soil in 5 parts water by hand and inserting a pH probe into the suspension.

To use a field pH test kit, a small sample of soil is collected and an indicator solution is added to form a paste. The paste is then coated with barium sulphate powder. The powder changes colour depending on the pH of the soil, and the colour of the powder is compared with a colour chart. This method is not as precise as the laboratory methods. The kits deteriorate with age and therefore should be discarded after 12-18 months.

Related Link

Further information about Surface Soil pH is available within the Soil section of VRO.
Further information about the Nitrogen cycle is also available.