solum

Solum is a term used in soil science to denote the upper part of a soil profile that has been altered by soil-forming processes, or pedogenesis. It generally encompasses the horizons that have undergone weathering and organic and mineral transformations, typically the A horizon, any E horizon, and the B horizon, along with their transitional layers. The organic O horizon is sometimes included, depending on the convention, while the C horizon and unaltered parent material are not part of the solum.

In practice, the solum represents the portion of the soil profile that has experienced the same soil-forming conditions across its thickness. It is where the main pedogenic processes—such as horizon development, clay translocation, organic matter accumulation or decomposition, and mineral transformations—have altered the material from its original state. The depth and thickness of the solum vary with climate, parent material, and land use, and it may be disrupted by erosion, tillage, or bioturbation. The solum is typically the interval where most plant roots are concentrated and where the majority of nutrient cycling occurs.

Solum contrasts with regolith, the broader unconsolidated material above bedrock that includes both altered and unaltered zones. In soil classification, the solum is a key concept for assessing pedogenic development; soils with a thick, well-developed solum generally show stronger horizon differentiation and leaching patterns than soils with a shallow or weakly developed solum.