Soil Types: Acidifying Soils

So that ericaceous plants like camellias, blueberries, heathers, and rhododendrons can grow, acidifying garden soil will lower its pH. In most cases, it is only necessary if the soil’s pH is neutral or alkaline. The most prevalent acidifying substance is sulphur. The use of peat is no longer advised.

When To Acidify Soil

Products containing sulfur take longer to work when the soil is cold, so they are typically best applied from spring to autumn. However, soil-acidifying materials can be applied at any time of the year.

You need to check the pH of your soil to determine how much acidifying material you need to add, if any, before doing so.

The pH of the soil is determined by performing a pH test. A pH of 7.0 is thought to be neutral. The soil is alkaline above pH 7.0, and acid below pH 7.0. For more information, see our page on soil pH testing.

When ericaceous plant growth is disappointing or shows signs of chlorosis (yellowing of the leaves), it is especially important to test the pH of the soil before designing or planting a new garden with ericaceous plants.

Testing can be done at any time, but the results may be erroneous if they are taken within three months of adding lime, fertiliser, or organic matter.

If the results of your soil pH test are lower than 7.0, you already have acidic soil. However, if you want to grow ericaceous plants—those that dislike lime—you may need to further acidify the soil to a pH of between 5.0 and 6.0. If the pH of other plants is significantly lower than 6.5, you might want to add lime to raise the pH.

Acidifying Materials

Sulphur

The most prevalent acidifying substance is this. Sulphur is transformed into sulphuric acid by soil organisms, acidifying the soil. The sulphide can be converted by bacteria more quickly the finer it is ground; Sulphur dust has a shorter half-life and is more expensive than sulphur chips. Sulphur-induced acidification, on the other hand, takes weeks to work, and if the soil is cold in the winter, it might take months.

Even though sulphur is the least likely to harm plants and the cheapest acidifier, other materials are sometimes used.

Aluminium Sulphate

This is used as a “blueing agent” for hydrangeas in areas where the soil isn’t acid enough to produce blue flowers naturally. As a soil acidifier, aluminium sulphate can also be utilised. Although the effects are quick, large quantities may also cause excessive pH reduction and interfere with phosphorus levels in the soil. Additionally, repeated application can lead to a toxic build-up of aluminium in the soil.

Aluminium sulphate is required seven times more than sulphur. Despite being more costly than sulphur, the price is comparable to that of ferrous sulphate.

Ferrous Sulphate (sulphate of iron)

This provides iron and has an acidifying capacity comparable to that of aluminium sulphate. Ferrous sulphate acts quickly because it immediately dissociates into iron ions that bind to the soil’s clay, displacing hydrogen ions that remain in solution with the sulphate ions—which are actually very dilute sulphuric acid—when washed in a lot of water.

However, when used in large quantities, it may impede phosphorus availability. After the acidification process, any signs of phosphorus deficiency can be corrected by watering with monopotassium phosphate.

Sulphate of iron is available in most garden centres, but because you need eight times as much as sulphur, it may cost more.

How To Acidify Soil

Quantities To Apply

Because of their capacity to buffer, clay-rich soils require significantly more sulphur than sandy soils to alter their pH. Because organic matter also functions as a buffer, soils with a high organic matter content will require more sulphur than those with a low organic matter content.

Soils that are very alkaline will need a lot of sulphate. The soil cannot possibly be acidified if it contains free chalk or lime. A soil sample can be tested for free carbonate or lime by adding vinegar. If there is “fizzing,” there is free calcium carbonate present.

Depending on whether the soil is sandy (lower figure) or clay (higher figure), sulphur powder may be required at a rate of 135-270g per sq m (4-8oz per sq yd) to change the pH of the top 15cm (6in) of the soil from neutral (pH 7.0) or slightly alkaline (pH 7.5) to slightly acid (pH 6.0-pH 6.5). Depending on whether the soil is sandy (lower figure) or clay (higher figure), ferrous sulphate may be required at a rate of 1080-2160g per sq m (32-63 oz per sq yd).

Sulphur may need to be incorporated into the soil down to at least 30 centimetres (1 foot), which is much more laborious and costly because some tree and shrub roots penetrate deep into the soil. It’s better to be safe and add small amounts over several months rather than one large dose because making the soil too acidic can be very bad for plants. Be open to trying new things.

Methods Of Application

Apply the required amount of sulphur to the soil that needs to be treated. Because the dust is very fine and easily drifts, do this in still weather. When treating large areas, it is sensible to wear gloves, goggles, and a dust mask.

Sulphur is most effectively incorporated into the soil through cultivation prior to planting so that it has ample time to work. When applied to the surface, changing the acidity at the root level can take years. If trees and shrubs with deep roots are to be planted, half of the soil may need to be dug up, and the other half may need to be cultivated by hoeing, raking, and cultivating. Where one is available, a rotovator is ideal.

Problems

Acid or alkaline soil conditions can be linked to certain plant nutrient deficiencies.

Plants That Need Acidic Soil

The majority of turf grasses and landscape plants prefer a slightly acidic soil pH of about 6.5. However, there are some plants that thrive best in soil that is more acidic.

Iron chlorosis, a deficiency that causes the veins or entire leaves to turn yellow, is often a sign that a plant needs a soil that is more acidic than the one it is growing in. If this occurs, your plants may require that you either add a soil amendment to the soil they are growing in or use an acid-supplementing fertiliser to meet their acid needs.

Acid-loving plants can get this help from a wide range of fertilisers and soil amendments. The majority are readily available at home improvement and garden centres.

Have your soil tested to determine the types and quantities of necessary soil amendments prior to application.

The following plants like acidic soil:

• Azaleas & Rhododendrons
• Camellias
• Ferns
• Magnolias
• Hydrangea
• Japanese Maples
• Cornus
• Hamamelis & Loropetalum -Witch Hazel
• Blackcurrants
• Blueberries

Frequently Asked Questions