Soil pH

What is soil pH?

pH (Potential of Hydrogen) is a numerical scale to measure how acid or alkaline a substance is. The pH values range from 1 to 14. Values below 7 are considered acidic, values above 7 are alkaline; and 7 is considered neutral.

The number is determined by the amount of hydrogen ions in the soil / substance. As the amount of hydrogen ions in the soil increases, the soil pH decreases, thus becoming more acidic. From pH 7 to 0, the soil is increasingly more acidic, and from pH 7 to 14, the soil is increasingly more alkaline or basic.

Most plants grow best when the pH is a slightly acidic between 6.5; and most vegetables prefer a slightly acidic soil with a pH of 6 to 6.5.

Why soil pH is important?[1]

Soil pH is important because it influences several soil factors affecting plant growth, such as soil bacteria, nutrient leaching, nutrient availability, toxic elements, and soil structure.

Bacterial activity that releases nitrogen from organic matter and certain fertilizers is particularly affected by soil pH, because bacteria operate best in the pH range of 5.5 to 7.0.

Plant nutrients leach out of soils with a pH below 5.0 much more rapidly than from soils with values between 5.0 and 7.5. Plant nutrients are generally most available to plants in the pH range 5.5 to 6.5.

Aluminium may become toxic to plant growth in certain soils with a pH below 5.0.

The structure of the soil, especially of clay, is affected by pH. In the optimum pH range (5.5 to 7.0) clay soils are granular and are easily worked, whereas if the soil pH is either extremely acid or extremely alkaline, clays tend to become sticky and hard to cultivate.

A test kit will give you an idea of your soil pH. However, these tests don’t take soil type into consideration. We will discuss soil type next time.

Nutrients availability[3]

Acid soils with a pH of less than 6 may have deficiencies in magnesium, calcium, magnesium, phosphorus, potassium and molybdenum.

Acid soils with a pH of less than 4 commonly have toxic amounts of aluminium and manganese.

Alkaline soils with a pH of more than 7 the following nutrients may be unavailable: iron, manganese, zinc, copper and boron.

Adjusting the pH will make these nutrients available to your plants. Organic matter will generally ‘buffer’ plants against the impact of acidity so that a soil with a lower pH range will still successfully grow plants. Plants vary in their desired pH range and this is to do with the pH of the soil type they evolved in. You can find list of plants based on its preferred soil pH here.

Altering soil pH

Raising pH

If the soil is too acid, the most commonly used technique to raise the soil pH is applying agricultural lime (calcium carbonate). The amount needed will vary depending on the pH and soil type. As a rough guide apply 120 g/m2 to a clay soil and 30 g/m2 to a sandy soil. Test again in a few months and apply more if necessary. You can also use poultry manure.

Lowering pH

If the soil is alkaline, the technique to lower the soil pH is by adding things like compost and manures, leaf litter and mulch. Iron chelates work too. In extreme situations, powdered sulphur can be used – one handful per square meter, once a year. Sulphur works very slowly and a change won’t be noticed for about 6 months.


(image courtesy of



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