Soil salinity changes over 24 years in a Mediterranean irrigated district

Soil degradation from salt accumulation, sodication, or both, is a threat or a fact in many irrigated lands. Salinization has often been assessed from changing cropping patterns over time, and often the trends in salinization have not been quantified. Our objective was to identify trends in salinization or desalinization by direct measurements of soil salinity where a consistent methodology was maintained over time. The soils of the Flumen irrigation district (27,500 ha) in Aragón, Spain, were sampled in 1975. The same plots were sampled again in 1985/1986 and in 1999. There were 140 sampling points in 1975, and 66 in each of the other two surveys. The mean sampling depth was 103 cm, resulting in 909 soil samples and 8603 analytical determinations. Analytical results for salinity, individual ions, and pH retrieved from the first survey were compared with the two subsequent surveys. The electrical conductivity of the saturated extract (ECe) and the sodium adsorption ratio (SAR) of the extract were determined through the soil profiles, allowing us to compare the results from the three surveys. The upper meter of the soil was less saline in 1999 than in 1975. The median ECe of non-saline soils changed only slightly, while in the saline areas the median ECe for comparable soil depths averaged over 1 m was 5.9 dS m−1 in 1975, 3.1 dS m−1 in 1985/86, and 1.9 dS m−1 in 1999. The median of the maximum SAR to the same depth also decreased from 22.0 (mmol/L)0.5, to 15.1 (mmol/L)0.5, and to 10.5 (mmol/L)0.5 for the same three periods. Thus, soil salinity in the upper meter of soil has decreased during the last 24 years.