DEVIATIONS FROM THE EMPIRICAL SODIUM ADSORPTION RATIO (SAR) AND EXCHANGEABLE SODIUM PERCENTAGE (ESP) RELATIONSHIP.

Accurate information on exchangeable sodium percentage (ESP) is important in reclamation of sodium (Na)-affected soils. Representative soil samples were collected at six depths (0-5, 5-15, 15-30, 30-60, 60-90, and 90-120 cm) from five sites in the Powder River Basin (PRB), Wyoming, that have been irrigated with salinesodic water [electrical conductivity of irrigation water (ECiw) >2 dS m^-1 and sodium adsorption ratio of irrigation water (SARiw) >20 mmol1/2 L-1/2] for up to 3 years. Soil ESPs were determined using NH4OAc (ESPm) and the ESP-SAR empirical relationship (ESPe) recommended by U.S. Soil Salinity Laboratory Staff. Measured ESPm values were significantly lower (P < 0.05) than ESPe determined by the empirical relationship in the upper 30 cm; except for site 2, all remaining profiles had no statistically significant relationships for 30- to 120-cm depth samples. Significantly lower ESPm in these soils could be due to a combination of overcorrection for soluble Na because of salinity and underestimation of exchangeable Na due to fixation of the NH4+ index ion by smectitic clay minerals present in the PRB soils. Thus, using an appropriate extractant to determine the exchangeable Na and using a correction factor for accurate evaluation of soluble Na concentrations are important for developing suitable amendment plans for correcting sodicity hazard in saline-sodic waters used to irrigate soils of the PRB region.