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Soil pH (acidity) explained
and what to do about it

How to change the acidity (pH) of a soil

The acidity of a garden soil (referred to as its 'pH') is measured on a scale 0 to 14, with 7 being neutral (i.e. neither alkaline or acid), an alkaline soil has a pH value above 7, a value below 7 indicates an acid soil. In simple terms, the acidity reflects the amount of calcium (i.e. chalk or lime) in the soil, this changes over time partly as it is leached out of the soil by rain etc. and also by growing vegetables, using fertilisers and manures; all this leads to increased acidity as time goes by.

Clay soils hold calcium better than sandy soils where the calcium may be leached out relatively quickly, the natural tendency is for clay soils to be alkaline (pH above 7), while sandy soils tend towards acid (pH below 7).

The importance of the pH is that it affects the release of nutrients and worm/micro-organism activity and, high acidity, increases some plant diseases.

The pH of a garden soil has an affect upon plant growth; generally vegetables like a soil with a pH of around 6.5 (i.e. slightly acid) but there is no common pH preference for shrubs and flowers etc, some don't mind, others desire less acidity while other desire more (some plants are often classed as 'chalk/clay loving or hating', this just reflects the pH preference of the plant).

A vegetable plot is likely to need more attention regarding the soil pH than flowers or shrubs.

The pH of soil can be easily determined by using one of the readily available soil sampling kits; usually the method is to add a chemical to a mixture of the soil and water, the colour change of the sample is then compared to a chart provided and the colour match identifies the pH of the sample. It is recommended that two or three soil samples from different areas are tested to avoid a possible unrepresentative test result.

How to change the acidity (pH) of a soil

To decrease the acidity, add lime - the preferred method is to use ground limestone or chalk. The amount required will depend upon the soil type and the degree of increase in pH desired. Lime should not be added at the same time as fertiliser or organic material (they can react).

If both organic material/fertiliser and lime are required, add the organic material/fertiliser first (in Autumn) and the lime in February/March. If just liming is required, it should be applied to chalk/clay soil in Autumn or to sandy soils in Spring.

Lime is traditionally spread on the surface of the soil for rain to wash it in but on medium to heavy soils, it is better to dig it in to the top spit of soil.

The amount of liming necessary will depend upon the type of soil, the actual pH and the desired pH; the wider the gap (for a given soil type), the more liming required. As a very rough guide, to take a soil to 6.5 pH the suggested liming (using ground limestone) is:

Current pH
Sandy soil
g/sq m (oz/sq yd)
Loamy soil
g/sq m (oz/sq yd)
Clay soil
g/sq m (oz/sq yd)
6.0
140 (4)
200 (5)
240 (8)
5.5
240 (8)
380 (11)
480 (14)
5.0
440 (13)
620 (18)
820 (24)

Lime should not be required each year, with vegetable crop rotation every three or four years should be adequate.

To decrease the acidity, add compost or other organic material.

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