 Bulk soil

Bulk soil is the surrounding or reference soil sampled separately from root-associated material in a specified experiment. Its meaning is operational: the sampling distance, soil layer and treatment history determine the comparison. A2012 Arabidopsis thaliana study used parallel pots without a plant as bulk soil controls when comparing root-associated bacterial communities.^[ref]

An invented example illustrates why the reference matters. Suppose an assay gives6 units in bulk soil A and10 units in its root-associated sample, but26 and30 units for the corresponding samples from soil B. Both within-soil differences are4 units. Comparing10 directly with30 would mix the difference in starting soil with the root-associated contrast. The example is a comparison of additive assay responses, not a claim about a measured bacterial population.

Bulk soil does not mean biologically empty soil, nor does it guarantee a history completely free of roots. It names the reference selected for the stated question. For a useful comparison with the rhizosphere, report the sampling rule and measured quantity as well as the label.