Evaluation Of Soil Sustainability Along The Rio Grande In West Texas: Changes In Salt Loading And Organic Nutrients Due To Farming Practices

Growing populations demand an increase in the amount of food being produced, which in turn, puts pressure on the productivity and sustainability of soils. The use of flood irrigation from the Rio Grande has greatly increased the salinity, sodicity, and enhanced leaching of nutrients in the Rio Grande Basin soils. To evaluate soil health, water and soil samples were collected from two pecan, one cotton, and one alfalfa field in western Texas. Each site was equipped with sensors to measure soil moisture, temperature, and electrical conductivity (EC), along with lysimeters at depths of 15, 30, and 60 cm to collect soil water samples. Soil solution, irrigation water, and drainage water samples were collected during the 2011 irrigation season and analyzed for pH, EC, major elemental chemistry. Soils at each site were collected before irrigation in 2011 and characterized for soil salinity, sodicity and alkalinity, as well as nutrient levels. The water used for irrigation is high in TDS, and especially enriched in dissolved Na+, Cl-, Ca2+ and SO42-. With water penetration during flood irrigation, soil water chemistry evolves by dissolving salts in the soils such as gypsum and halite. These salts almost reaches saturation with respect to these two evaporate minerals at 60 cm below ground surface. In contrast, soil water becomes oversaturated with respect to carbonates, in agreement with ~10wt% of pedogenic calcite observed throughout the soil column. The drainage water is chemically similar to the irrigation water, suggesting that most of the irrigation water has a shallow drainage path. Between irrigation, however, soil water become more concentrated as water is lost through evapotranspiration. Thus, it is reasonable to assume that the salts precipitate out at this stage. Soil sodicity and salinity increase with depth in cotton and pecan sites and are observed to be over the tolerance levels of cotton and pecan at greater than 30 cm. These observations are in agreement with the facts that pecan orchards are more intensively irrigated and thus have higher salt loading, and that Cotton soils contain a higher clay content. Alfalfa soils however, are generally less sodic. Sodic soils are known to have lower permeability, supporting the observation that irrigation water does not penetrate to deeper soils but flows to drainage canal instead. Salinity affects crop growth by limiting the availability of water and nutrients. This study demonstrates that the quality of crop soils in the Rio Grande Basin deteriorate over time by irrigation with high TDS Rio Grande water, addition of fertilizers, and other agricultural practices. Irrigation alone loaded approximately one ton per acre of Na, SO4, and Cl to the soils in 2011 irrigation season. Organics such as DOC are low in irrigation waters, and are loaded into the soil from leaf litter. Results from this project contribute to our understanding of salt loading and nutrient cycling in the vulnerable area of the Rio Grande Valley in West Texas