TEXTURE CONTROLS ON THE LOADING AND MIGRATION OF IRRIGATION-DERIVED SALTS IN THE RIO GRANDE AGRICULTURAL SOIL ALONG THE US-MEXICO BORDER

Intense irrigation can load salts and have adverse effects on the normal functioning of soils in arid and semi-arid lands. Salt can change soil texture, porosity, permeability and the behavior of clays, playing critical roles in the uptake of water and nutrients by crops, and influencing soil structural stability and overall crop yield. The goal of this study was to quantify the amount of salts loaded each year by means of irrigation, to determine what factors control salt mobility and transport, and to evaluate the effectiveness of gypsum application to lower sodicity in soils of the northern Chihuahuan Desert, near El Paso, TX.

A hand auger was used to extract soil from a pecan farm at about 10cm intervals in depth for a total of 3m in February 2012. After collection, each sample was characterized for CEC, pH, SAR, exchangeable and water soluble cation levels following standard extraction methods. Particle size distribution was obtained through the Malvern Mastersizer 2000 laser diffractometer for particles less than 2mm. This work is complementary to a study where similar soil data was collected before the growing season of 2011 in the same field. However, 10 tons/acre of gypsum was added to the field in January 2012.

Results indicate that both exchangeable and water soluble cation concentrations as well as total CEC, are dependent on soil texture: highest in clayey soils and lowest in more sandy soils. Furthermore, the texture change observed justifies the common practice that sandy soils are mixed with clayey soils to improve the soil texture and structure in west TX.

Ca and Na were most dominant with values ranging from 0.22 to 3.1 cmol(c)/kg for water soluble ions. Mg and K also contributed to overall charges but concentrations were much lower than Ca and Na. SAR values observed in 2011, ranged from 4 to 13. 2012 values were found to be between 3 and 9, still under the sodicity limit of 13. Reduced SAR values from 2012 suggest that gypsum added by farmers is effective in reducing soil sodicity.

We showed that salt loading made the soils alkaline, saline and sodic. Agricultural practices such as plowing and gypsum addition can improve soil quality but will not sustain quality and health long term. Future work will examine salt loading at different locations in the Chihuahuan Desert to obtain an analysis of the region with better spatial scales.