Bulk density
The dry mass of soil per unit bulk volume of soil, expressed as g/cm3, usually given on an oven-dry (110° C) basis.
Base saturation
The degree to which a soil having cation-exchange properties is saturated with exchangeable bases (sum of Ca, Mg, Na, K), expressed as a percentage of the total cation-exchange capacity.
Aggregation
Process whereby primary soil particles (sand, silt, clay) are bound together, usually by natural forces and substances derived from root exudates and microbial activity. Soil aggregates are arranged to form soil peds, units of soil structure, classified by shape, size or grade. Definitions and size classes can be found in Six et al. (2004).
Aggregates
Soil aggregates are soil peds of a range of sizes (microaggregates 53-250μm and macroaggregates >250μm (usually taken as 250-2000μm)). Definitions and size classes can be found in Six et al. (2004).
Topsoil
The surface soil horizon (A) which is modified when cultivated, and designated Ap.
Subsoil
Technically, soil horizons below plough depth; usually B horizons.
Solum
Topsoil and subsoil layers that have undergone the same soil forming conditions. The base of the solum (plural, sola) is the relatively unweathered parent material. Solum and soils are not synonymous. Some soils include layers that are not affected by soil formation.
Soil
A combination of four constituents: mineral material (sand, silt, clay and rock particles), organic material, air and water, forming a natural, three-dimensional body at the earth's surface. It is capable of supporting plant growth and has properties resulting from the integrated effect of climate and living matter acting on parent material, as conditioned by relief [...]
Soil depth
Depth of soil profile from the top to parent material or bedrock or to the layer below root penetration is not (or no longer) possible. It differs significantly for different soil types. It is one of basic criterions used in soil classification. Soils can be very shallow (less than 25 cm), shallow (25 cm-50 cm), [...]
R horizon
Hard, consolidated bedrock beneath the soil. The bedrock commonly underlies a C horizon but can be directly below an A or a B horizon.