The pH of a soil indicates how acid or alkaline that soil is; it is measured on a numerical scale of 1 – 14. The acidity and alkalinity of a soil affects the plants which may be successfully grown in that soil.

The scale reads 7 as neutral; with the low numbers 1 – 7 as an acid; whilst 7- 14 is alkaline. You may remember using the pH scale in chemistry lessons, but you may have forgotten that it is a logarithmic scale. What this means is that pH 5 is ten times more acid than pH 6

Soils in the UK naturally range from 4 to 8, so are more likely to be acidic than alkaline, although very acidic soils are not common outside the peat bogs.

The initials pH stand for ‘potential for hydrogen’. Technically, this refers to the number of hydrogen ions in the soil. The more hydrogen ions there are, the more acid the soil. Somewhat perversely, the number of hydrogen ions in the soil increases as the pH decreases. So there are ten times more hydrogen ions in pH 6 than in pH 7.

Testing whether you have acid soil or alkaline soil in your garden soil is helpful as it helps you to learn more about which plants will grow well in the soil you have. It forms an essentials part of the garden site survey we carry out during the garden design process. Simple pH meters will give you a general idea although taking soil samples and using a soil testing kit bought from a garden centre is more accurate than using the meter.

It is advisable to test many areas in your garden as the results may surprise you! Testing the soil is one of the elements in the first gardening lesson on Plews’ Basic Gardening course and students nearly always find the results fascinating.


pH meter, acid soil, alkaline soil, measuring pH of soil

Chartered Institute of Horticulture