Soil pH


Knowing your soil type and whether it is acidic, neutral or alkaline will help you choose the right plants for your garden and how to treat your soil.

Soils usually range from pH 4 to 8, and most plants prefer a range of 5.5 to 7.5.

Some plants only survive in acid conditions while others prefer alkaline soil.

Varying levels of pH can affect soil texture, distribution of nutrients, and the tiny organisms that dwell in the soil.

The term pH refers to the acidity or alkalinity of soil,where soil acidity is measured in pH units on a scale of 1 to 14, and it refers to the amount of hydrogen (h) in the soil.

A pH of 7.0 is taken as the central, neutral point (true neutral) i.e. it is neither acid nor alkaline, although, horticulturally, neutral soil is pH6.5.

Put another way, pH measurements from 7.0 to 0 are increasingly acid, and from 7.0 to 14 more alkaline.

To put this into perspective: a drop of one point on the scale denotes the soil has increased in acidity by ten times, conversely, a one point drop means it is ten times more alkaline.

Extremes below pH 5.5 (acid) and above pH7.5 (alkaline) can be problematic, with certain pests, diseases and nutritional disorders becoming more prevalent.

For example:

Magnesium deficiency and clubroot are more prevalent on acid soils, while trace element deficiencies are common on alkaline soils.

A very alkaline soil can cause nutrients to be locked into the soil.

Soil types:

Soils can become more acid over time, particularly in wet climates.

One should aim for pH 5.5 to pH 7.5.

Soil testing:

Tests should be made annually, and basically there are two ways to do this!

Quite often when taking this route, the laboratory will give information other than the pH, i.e. such things as the NPK content and amount of trace elements held in the soil.

Some laboratories will even advise you the best course of action to take to improve the quality of your soil.

  • Buy a D.I.Y. test kit.

Follow the instructions on the packaging, and identify the pH by using the enclosed / attached colour chart.

Soil Testing Kit

After the tests use the following guides


For example to make your soil more alkaline by 1 pH ;

To make your soil more acid by 1 pH

General Information:

To do so may create a harmful reaction.

If you normally apply manure in the autumn then leave liming until early spring, say about six weeks before planting.

Over-liming is difficult to correct, add less than the recommended amounts at first, and adjust later after it has had time to weather in, or wait till the following season.

Top of the Page