Soil properties within the Rio Grande valley near El Paso are strongly linked to the types of fluvial deposits that serve as parent material. We used four geophysical techniques (DC resistivity, ground conductivity, capacitively coupled resistivity and magnetics) to distinguish between soil units in an alfalfa field. We combined these observations with geochemical characterization and particle size analysis in order to determine how these soils and irrigation practices influence salt buildup and water availability, and thus crop growth. Soils mapped at the site were derived from crevasse splay and flood plain deposits. Results of our investigation showed that the alfalfa grew better in soils with a 1.25 m thick unit of 40-70% sand that fined into a silty-clay (<20% sand) at greater depths. Poorer growth occurred in soils where a 0.8 m thick silty-clay (<20% sand) was underlain by a less than 2 m thick sand unit (>90% sand) we interpret as an abandoned river channel. The DC resistivity, capacitively coupled resistivity and conductivity surveys were all responsive to the major grain size changes in the upper 3 meters of soil and were able to distinguish the buried river channel. The magnetics survey was not as successful at detecting the channel, but was able to characterize near-surface grain size variability and hence distinguish between the major soil units found at the site. We believe that similar geophysical techniques could be used to rapidly evaluate soil characteristics in other regions where soils are derived from fluvial parent material.

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