Criteria for Defining the Soil Family and Soil Sibling

Book excerpt: Soils need to be classified so they can be identified and mapped. The classification of soils in New Zealand is described in two publications in the Landcare Research Science Series: The New Zealand Soil Classification (Hewitt 2010) and this new report by Webb and Lilburne, Criteria for defining the soil family and soil sibling. The New Zealand Soil Classification classifies NZ soils into 15 soil orders, each of which is divided successively into soil groups and subgroups; this provides an important framework to understand the occurrence and broad properties of soils in the landscape. Criteria for defining the soil family and soil sibling defines the next two levels of classification: soil families and siblings. The sibling is the primary entity depicted on soil maps. Families and siblings separate soils into well defined classes that define each soil's physical composition. The family criteria separate soils on the basis of three criteria: the nature of the soil profile material to 1.0 metre depth, the dominant texture in the upper 0.6 m, and minimum permeability within 1.0 m depth. The sibling criteria separate soils mainly according to the composition of horizons that make up the soil profile.