Soil pH – small numbers, big consequences

A difference of one unit in soil pH might not seem significant when reviewing soil test results. However, because the pH scale is logarithmic, every drop of one pH unit represents a tenfold increase in hydrogen ion activity, a measure of soil acidity. Veronica Nyhan, Teagasc Soil and Plant Nutrition Specialist, tells us more.

For example, a soil at pH 5.5 is ten times more acidic than one at pH 6.5. Compared with neutral pH 7.0, a soil at pH 5.3 has approximately 50 times the hydrogen ion activity, increasing to 100 times at pH 5.0.

Phosphorus (P) is particularly sensitive to soil pH. On acidic mineral soils, P increasingly reacts with iron and aluminium, forming compounds that are less available to crops.

As soil pH falls, particularly below 6.0, phosphorus availability can become increasingly restricted.  Although phosphorus may be present in the soil, a proportion can be held in forms that are not readily available for crop uptake.  Correcting soil acidity through liming can improve phosphorus availability, allowing crops to make better use of both existing soil phosphorus and applied fertiliser.

For most cereal crops, the target soil pH is approximately 6.5, while beans and oilseed rape require closer to 6.8.

Review recent soil test results and identify fields where pH is below target. Follow the laboratory lime recommendation when determining application rates, rather than relying on soil pH alone. Two soils with the same pH can have very different lime requirements because of differences in soil buffering capacity.

Where conditions are suitable, apply lime evenly and incorporate it into the cultivated layer where appropriate. Avoid spreading on waterlogged soils or where there is a risk of runoff.  Lime takes time to react with soil, so correcting low pH should be planned well in advance of the next crop.

Before investing in additional phosphorus fertiliser for 2027, ensure that soil pH is being managed appropriately. Correcting soil acidity is an important first step in improving nutrient use efficiency and getting better value from fertiliser inputs.