USING GEOPHYSICS TO INVESTIGATE SOIL TEXTURE AND CHARACTERIZE SPATIAL VARIABILITY IN SOIL CO2 EFFLUXES IN AN IRRIGATED PECAN ORCHARD IN THE RIO GRANDE VALLEY OF WEST TEXAS

Agriculture in arid lands, such as in the Rio Grande Valley in west Texas, relies on flood irrigation. However, flood irrigation can cause salt buildup and greenhouse gas emissions associated with pedogenic carbonate precipitation. The ability of irrigation to promote crop growth is influenced by geochemical and hydrological processes controlled, in part, by soil grain size, larger-scale soil structure, and mineralogy. A pecan orchard in Tornillo, TX, with soil derived from fluvial deposits, undergoes flood irrigation every 2 to 3 weeks in spring through fall. This study utilized shallow geophysical methods (e.g., ground conductivity, magnetics, seismic, and resistivity) for 1) measuring various soil properties (soil texture, salt buildup, water table depth, and water flow patterns), 2) determining how texture and structure influence soil changes (e.g., salt buildup, pedogenic carbonate buildup, moisture retention) during irrigation cycles at the pecan grove, and 3) comparing the variability of these properties to understand the impact of agricultural practices on soils and crop growth. Soil texture firmly controls the physical, hydrological, and chemical properties of soils. Conductivity data indicate a correlation between high conductivity areas at 6 m depth to poor tree growth areas, suggesting high conductivity clays reduces water drainage, increases soil salinity, and limit root growth in parts of the field. Spatial surveys monitor the magnitude of soil CO₂ effluxes, suggesting that lower CO₂ rates correspond to larger trees and coarser soil. These results suggest that shallow geophysical techniques help provide insights into soil properties controlling the salt buildup, CO₂ emission, and plant growth in flood irrigated fields.