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


About mistralhowto

formulation chemist

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s