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Alum

Alum (Potassium Aluminium Sulphate)

Synonyms: Potash Alum, Potassium Alum
Chemical Formula: KAl(SO₄)₂·12H₂O
Common Sources: Natural minerals (Alunite, Bauxite), Industrial synthesis

Botanical and Historical Context

Alum is not a plant-derived substance but a naturally occurring mineral salt historically used in horticulture, medicine, and dyeing. It has been utilized for centuries as a soil amendment, astringent, and mordant in fabric dyeing.

Occurrence in Nature

Alum is commonly extracted from alunite (KAl₃(SO₄)₂(OH)₆), a sulphate mineral found in volcanic and sedimentary rock formations. It can also be derived from bauxite, a primary ore of aluminum.

Uses in Botany and Gardening

  • Soil Acidifier: Alum can be used to lower soil pH, making it beneficial for acid-loving plants like hydrangeas, azaleas, and blueberries.
  • Color Enhancement in Hydrangeas: The application of alum-based solutions can shift hydrangea blooms from pink to blue by increasing aluminum availability in acidic soil.
  • Preservative in Botanical Preparations: Historically used to preserve plant specimens and herbal extracts.

Chemical Properties

  • Solubility: Highly soluble in water, forming an acidic solution.
  • Astringent Effect: Causes tissue contraction, used medicinally and in tanning leather.
  • Antiseptic Qualities: Has mild antimicrobial properties.

Toxicity & Safety Considerations

  • Alum is generally safe in small quantities but can be irritating to skin and mucous membranes.
  • Overuse in soil can lead to excessive aluminum accumulation, which may be toxic to plants.
  • Not to be confused with ammonium alum or sodium alum, which have different applications and properties.
DALL·E 2025 02 15 08.43.42 A scientific botanical illustration of alum Potassium Aluminium Sulfate. The image features a detailed crystalline structure of alum with translucen

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