101 Questions answered
Lime Neutralises Acidity; Gypsum Improves Structure but Won’t Fix pH
If the goal is to raise soil pH, use lime. Gypsum can have a very slight but inconsistent impact on pH and can make molybdenum deficiency worse.
Lime (calcium carbonate) works because the carbonate reacts with hydrogen to neutralise soil acidity. When lime gets wet, it separates into calcium and carbonate ions. The calcium ions displace hydrogen ions from the soil particles into the soil solution. The carbonate ions then react with the hydrogen ions in the soil solution to form bicarbonate, which reacts with more hydrogen ions to form carbon dioxide and water. The end result is fewer hydrogen ions in the soil solution (and therefore higher pH), more calcium on the soil surfaces, and some carbon dioxide release.
Gypsum (calcium sulphate) dissolves into calcium and sulphate, neither of which neutralises hydrogen ions. While the calcium can also displace some hydrogen ions, there isn’t the carbonate to neutralise them, and hence there is little impact on soil pH.
It’s the carbonate vs sulphate part of the lime or gypsum that affects soil acidity.
Gypsum is better used to treat dispersive soil and as a source of sulphur for canola.
