Aluminium Sulphate Powder (10 mesh)- Soil pH Reducer, Molluscide, Hydrangea colour changer


SKU: RD10287

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ALUM - Technical Grade 10 mesh (2mm)


Aluminium sulphate, commonly known as alum is a crystalline powder that is soluble in water. It contains around 17% alumina (Al2O3). It is manufactured in a simple process from aluminium trihydrate and sulphuric acid. Aluminium sulfate has been used by man since 2000 BC, when the Egyptians used a mineral alum as a mordant in dyeing. It has long been used in paper sizing to improve durability and ink receptivity and in water treatment to clarify water. Other uses for alum include as a Ph adjuster in soil, as a molluscicide, as a waterproofing agent and accelerator in concrete.



  • Synonyms: aluminium sulphate; aluminium sulfate; Cake alum; Filter alum; Papermaker's alum; Sulfate d'aluminium;
  • Molecular formula: Al2(SO4)3.nH2O
  • Cas No: 10043-01-3
  • EINECS: 233-135-0
  • Molar mass: 342.15 g/mol
  • Appearance: white crystalline powder
  • Size: 10 mesh (2mm)
  • Density: 1.69 g/cm3
  • Melting point: 770 C decomposes
  • Solubility: soluble in water 43g/100mL (20C)
  • Refractive index: 1.47
  • Acidity (pKa): 3.3-3.6

It is slightly soluble in alcohols and dilute mineral acids.



Aluminium Sulfate is used in water purification. As a mordant in dyeing and printing textiles. In water purification, it causes impurities to coagulate which are removed as the particulate settles to the bottom of the container or more easily filtered.

Aluminium sulfate is sometimes used to reduce the pH of garden soil. The gardener can add Aluminium sulfate to the soil to reduce the pH level which in turn will result in the flowers of the Hydrangea turning a different colour (see below).

Aluminium sulfate is the active ingredient of some antiperspirants.

In construction industry it is used as waterproofing agent and accelerator in concrete.

In paper making the alum reacts with rosin sizes of various types, helping to attach the newly formed rosin aluminates to fibres. Sizing makes the paper water resistant.

It is also used in styptic pencils.

It can also be very 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.(See below - How to use aluminium sulphate to kill slugs)



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.


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. 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. 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.

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.


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.

Health & safety

Risk phrases: R36/37/38 Irritating to eyes, respiratory system and skin.

safety phrases: S26 In case of contact with eyes, rinse immediately with plenty of water and seek medical advice. S28 After contact with skin, wash immediately with plenty of: water. S37/39 Wear suitable gloves and eye/face protection. S38 In case of insufficient ventilation, wear suitable respiratory equipment.


PLEASE NOTE: This product is not for human or animal consumption.


Click here for MSDS

Click here for Specification Sheet