Basic Slag (Thomas Slag) – A Botanical and Agricultural Entry
Definition and Composition
Basic slag, also known as Thomas slag, is a byproduct of the steel industry, historically used as a phosphatic fertiliser in agriculture. It results from the basic oxygen steelmaking process, where iron ore is refined using lime to remove impurities, leaving behind a calcium silicate-rich residue with a significant amount of phosphorus.
Chemical Composition:
- Phosphorus (P₂O₅): 10–20%
- Calcium (CaO): 40–50%
- Silicon (SiO₂): 10–15%
- Magnesium (MgO), Iron (Fe), and trace elements
Agricultural Use
Basic slag was widely used in acidic soils as a slow-release phosphate fertiliser, particularly in grasslands, pastures, and arable crops. It neutralises soil acidity due to its lime content while supplying essential phosphorus for plant growth.
Benefits to Plants
- Promotes root development and flowering.
- Improves soil structure and microbial activity.
- Increases resistance to diseases in crops.
- Provides long-lasting phosphorus availability.
Application and Suitability
Basic slag is most effective in light, sandy, and peaty soils with a pH below 6.5. It is typically applied as a fine powder and incorporated into the soil before planting.
Decline in Use
The use of basic slag has decreased since the mid-20th century due to changes in steel production and the availability of water-soluble phosphate fertilisers. However, it remains of historical interest and is still used in certain organic and sustainable farming practices.
