Challenges Associated with Sampling Dynamic Soil Properties.

The determination of dynamic soil properties (DSPs) for agricultural practices poses significant challenges, particularly in the context of values derived as part of the National Soil Survey. Although DSPs have been defined as those properties that change over human time scales, limits on the time frame and scale of DSP change remain under discussion.  Some DSPs change over very short periods of time (days or weeks), others take decades.  In addition, DSPs are usually sensitive to horizontal and vertical spatial variation.  For example, surface soil bulk density (BD) is usually reduced after disturbance such as surface tillage and then responds to subsequent rainfall by consolidating.  The change after tillage is instantaneous but consolidation may take weeks. The spatial change in BD depends on the type of tillage tool, depth of tillage, etc. while the rate of change may be controlled by rainfall characteristics and inherent soil properties. Surface soil organic carbon (SOC) responds more slowly to tillage practices but differences in SOC distribution with depth are easily demonstrated.  In a few instances, measurement tools allow us to collect large amounts of DSP data that must then be interpreted at the appropriate spatial and temporal scale. Apparent electrical conductivity (ECa) is an approach for gathering spatially dense data that reflects spatial variability in soil properties. Correlations among ECa and inherent soil properties are usually strong when these properties are measured over the same depth as that sensed by the ECa instrument. Correlations among ECa and surface soil properties are usually poorer. We attribute this to management activities (e.g. tillage, fertilization, land leveling) modifying or disturbing spatial patterns in the surface layer. This presentation will examine the effects of spatial and temporal variation of dynamic soil properties in agricultural (cropped) systems, and discuss challenges associated with determining appropriate steady state values or ranges.