Lime is made up of a) calcium or b) calcium and magnesium and reduces the pH of soil (reduces acidity) It also has other properties that include:
a) binds to excess aluminum, manganese and iron that may – in excess – cause plant growth problems.
b) adds calcium and phosphorus
c) may feed some bacteria
There are two kinds of lime: calcitic and dolomitic.
Calcitic is pure calcium carbonate and is the cheapest and most readily available.
Dolomitic contains calcium carbonate but also an equal amount of magnesium carbonate.
The main use of lime in gardens is to reduce the acidity in soils.
Note that most vegetables and flowers grow best with a pH (acidity level) of between 5.5 and 7.0 (but there’s a lot of other factors that influence growth beside straight soil acidity.
Note: adding compost – and lots of it – masks a great many soil acidity issues.
Gypsum is calcium sulphate and it:
a) doesn’t reduce soil acidity
b) is used to “floculate” soil particles. In simple terms, when you add it to soils it attracts small clay soil particles together and makes them larger. This improves drainage and makes heavy clay soil into workable garden soil (albeit still clay)
c) it adds calcium and sulphur – two necessary plant growth elements.
So knowing that gypsum doesn’t reduce soil acidity, the main use for this material in our gardens is to improve clay soils.
How To Use:
Gypsum can be applied any time of year and in the home garden, one recommendation is to apply 40-50 pounds (22 kg) per 1000 square feet to established gardens with heavy clay soil. You can simply spread this and water it in. This is normally a one-time application and doesn’t need repeating.
Lime is a winter or very early application and the recommended application is between 20-30 pounds (9:13Kg) per 1000 square feet. Lime should be worked into the soil.