Electromagnetic Induction As a Tool for Mapping Coarse Fragments in Texas.

Presence of coarse fragments in soils complicates measurement of soil water content, and other soil properties, by physically hindering the insertion of probes, volumetric samplers, and other measurement tools.  Near San Marcos, TX, on a gravelly clay loam with woody encroachment, bulk apparent electrical conductivity (EC_a) was measured using the 0.5 m coil spacing of an EM38 four times during a soil dry-down period. Bulk density and coarse fragments were measured at field capacity for large soil volumes and soil water content was measured for each survey at 0-10, 10-20, and 20-30 cm. Sampling depths were chosen based on expected sensitivity of the EM38 and soil water contents were profile-averaged for each location. The visual pattern of the EC_a values across the site remained consistent over time. Bulk apparent electrical conductivity during field capacity decreased with increasing soil bulk density (r² = 0.64), and samples with increasing bulk density were associated with increasing coarse fragments (r² = 0.35). However, EC_a values were not correlated with soil water content for any of the four surveys. In areas with dense woody vegetation, EC_a tended to be lower with higher amounts of coarse fragments. Because of this, it may be possible to use woody vegetation cover as an indicator of the spatial distribution of coarse fragments in these soils.