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:

Clay or Sandy soils tend to be acidic.

Chalky soils are generally alkaline, except in areas of high rainfall.

Uncultivated Peaty soils are usually acidic.

Organic / cultivated soil with a high humus content is often more acid, as decomposition produces acidic by-products.

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!

1) By sending soil samples to a Soil laboratory.

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.

2) Buy a D.I.Y. test kit.

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

After the tests use the following guides:

To make your soil more alkaline by 1 pH ;

Apply 270 grammes of Ground limestone per square metre (8oz per sq yd) to sandy soils.

Apply 540 grammes of Ground limestone per square metre (16oz per sq yd) to loamy soils.

Apply 810 grammes of Ground limestone per square metre (24oz per sq yd) to clay soils.

To make your soil more acid by 1 pH

Apply 70grammes of Ammonium Sulphate per square metre (2 1/2oz per sq yd)

Adding garden compost and manure can help to make the soil more acid.


General Information:

It is very important not to apply lime at the same time as you are adding manure, compost or fertilizer.

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.

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