How to kill and control slugs with aluminium sulphate

Aluminium sulphate is effective as a molluscide, killing spanish slugs. Aluminium sulphate is considered to be a relatively environmentally friendly treatment for slugs compared to the more toxic conventional products like metaldehyde.

Aluminium sulphate is effective for killing slugs by desiccating them (drying them out by absorbing water from the slime producing glands). As well as killing young slugs it acts as a deterrent keeping them away from plants.
Apply the aluminium sulphate close to and around plants to be protected especially those most favoured by slugs, like Hostas. Apply the crystals at a rate of around 1kg per 40 square metres. Avoid touching the plants with the solid as this can cause them harm.
Note: Kibbled aluminium sulphate contains some very large crystals and these should be broken up into smaller pieces by placing inside a towel or other material and carefully hitting with a hammer.

Another method for applying aluminium sulphate is as a solution. This is highly favoured by growers of delphiniums and is best applied in the winter and early spring. Make up a solution by dissolving around 100g of aluminium sulphate in 5 litres of warm / hot water. Allow to cool. Apply around the base of the plants by saturating the soil. Be careful to avoid spraying directly on plants. Repeat every few weeks. As aluminium sulphate acidifies the soil it may be necessary to apply some lime on treated soil to raise the pH again.

Where to buy aluminium sulphate online in UK


How to reduce pH of soil with aluminium sulphate


The pH scale is simply a mesurement of the level of acidity. The pH scale is 1 to 14 and is nomally viewed in 3 sections: pH1 to pH6.9 is ACID, pH7 is NEUTRAL and pH7.1 to pH14 is ALKALINE (or BASIC).

Most plants prefer a slightly acidic soil between pH6 and pH6.8 and some plants like rhododendrons, azaleas and bluberries thrive in more acidic soils around a pH5 to pH5.5.


The aluminium sulphate should be applied to the garden as a liquid to realise a fast change in the pH. Before starting you need to know the pH of your soil. This can easily be done using a soil testing kit you can buy in any good garden centre or with a pH meter. As a rule of thumb to reduce the pH by 1 point you need to apply 50g of aluminium sulphate for every square metre of soil.

Example: if your soil pH is 7.5 and you want to reduce it to pH6.5 and you have an area of 30 square metres then you will need 50×30 = 1500g of aluminium sulphate.

Ideally the aluminium sulphate needs to penetrate into the soil to be effective or you will be just acidifying the surface. One suggestion to aid penetration would be to spike the garden as you go along. For every 5 square metres of garden you need to apply 250g of the aluminium sulphate. Dissolve this in a litre of hot water by leaving it to sit for 15–30 minutes. Add to a watering can and top up with cold water. Apply this to the 5 sq m of garden. Repeat the process for each 5 sq m to be treated.

Alternatively you could dig it into the soil again at a rate of 50g per square metre but this will take a lot longer to be effective as the sulphate is slow to dissolve.

Where to buy aluminium sulphate online in UK


Hydrangeas are fascinating in that, unlike most other plants, the color of their flowers can change dramatically.

It would be nice if one could change the color of hydrangeas easily. But for most of us, it is not easy. The people who have the most control over the color of their hydrangeas are those who grow them in containers. It is much easier to control or alter the pH of the soil in a container than it is in the ground.

In order to change the flowers of the hydrangea from pink to blue there needs to be aluminium present in the soil and a pH of less than 7 ideally between 5.2 and 5.5 (Acidic). Only plants older than 2 years which are established should be treated. Before commencing treatment to change colour it is strongly advisable to water plants thoroughly every day for a week. Where possible test soil around hydrangeas for pH.

Add 15g (1 tablespoon) of aluminium sulphate in a litre of warm water and allow to sit for 15-30 minutes to dissolve. Add this to a watering can and make up to 5 litres with cold water. Apply the solution around the based of the hydrangea. DO NOT OVERUSE as making the soil too acidic can result in damage to the roots. Check the pH and maintain it between 5.2 and 5.5.

Where to buy aluminium sulphate online in UK