To allow a comparison of the NABO soil samples, their density, percentage of fine soil and water content have been recorded for each sampling since 2003. These physical parameters help detect unusual conditions and avoid systematic errors. The condition during sampling influences the properties of the soil sample: Soil is a dynamic body that can swell or shrink dependent on the water content. The figure exemplifies, based on real data, how soil moisture can influence a soil sample.
At the same location, a soil sample from the top 20 cm is taken on two separate dates, first when the soil is dry, then when it is wet. With increasing water content, the soil material swells and each soil horizon expands. For a fixed sampling depth of 0 to 20 cm, this relationship means that dependent on water content different amounts of the various soil horizons are sampled. For soil parameters with pronounced depth gradients, such as organic carbon or lead, different concentrations are measured dependent on soil moisture.