Clay soils
can vary from pure clay to a mixture of clay, organic materials and sand.
The role of lime in amending clay soils depends not only on the density and pH of the soil but also on the type of lime used. There are five basic types of lime available, each with a different application.
Agricultural Lime and Dolomite
An extreme example of dry clay soil.
The role of agricultural lime or calcium carbonate and dolomite is to correct soil acidity. Dolomite is a naturally occurring rock that contains calcium carbonate and magnesium in a 2:1 ratio.
Burnt Lime and Hydrated Lime
For clay soils to be productive they must be easily broken up and have a balanced pH.
Burnt lime (calcium oxide) and hydrated lime (calcium hydroxide), commonly known as builders' lime, can modify the soil to create a more stable base for building. The application of these limes will reduce the plasticity of clay by decreasing water retention. These forms of lime are extremely caustic, however, and therefore are not used on garden soils.
Gypsum
Gypsum, shown here in its natural state, can improve clay soils..
Gypsum---or calcium sulphate---is sometimes associated with lime, but it is not lime and does not reduce soil acidity. It is of some benefit to clay soils, however, as it helps make the clay friable, or easily broken apart.
Application of Lime and Gypsum
Clay soils can be productive.
A desirable pH for most plants and vegetables is 6.0 to 7.0. Acidic clay soils will need more lime than acidic sandy soils. Gypsum is sometimes marketed in nurseries as "clay breaker" to improve clay soil structure.
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