Bulk soil is soil

Soil

Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

 outside the rhizosphere

Rhizosphere (ecology)

The rhizosphere is the narrow region of soil that is directly influenced by root secretions and associated soil microorganisms. Soil which is not part of the rhizosphere is known as bulk soil. The rhizosphere contains many bacteria that feed on sloughed-off plant cells, termed rhizodeposition, and...

. Bulk soil is not penetrated by plant root

Root

In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...

s. Natural organic compounds are much lower in bulk soil than in the rhizosphere. Microbial populations are typically lower in bulk soil than in rhizospheric soil. Furthermore, bulk soil inhabitants are generally smaller than identical species in the rhizosphere.