SOIL-STRUCTURE DEGRADATION AND MELLOWING OF COMPACTED SOILS BY SALINE-SODIC SOLUTIONS

Aggregates are formed by Joining of structural units of different size s in a hierarchical order. During wetting, the aggregates may either d isintegrate completely (slaking) or remain intact with only loosening at the points of weakness (mellowing). This study investigated the eff ect of saline-sodic solutions with sodium adsorption ratio (SAR) of 5 and 20 and electrical conductivity (EC) of 0.1 to 4 dS m(-1) after a n umber of wetting-drying cycles on soil mellowing, A Vertic Palexeralf with a coefficient of linear extensibility (COLE) value of 0.15 and a Typic Haploxeralf with COLE of 0.05 were studied. Using a Ca solution, the degree of aggregation in both soils was improved about two times in >50 and 20- to 50-mu m aggregates compared with the original soils, The values for the Vertic Palexeralf were three times those of the Ty pic Haploxeralf. In contrast, sodic solutions led to the collapse of a ggregates, Mellowing ratios of aggregates changed after 10 wetting-dry ing cycles and were approximate to 0.6 to 0.7 for SAR 5 and 0.8 to 1 f or SAR 20 for the Vertic Palexeralf; for the Typic Haploxeralf they we re 0.7 to 0.8 and 0.9 to 1.2 for SARs 5 and 20, respectively, Mellowin g ratios between 0.9 and 1 were obtained for the Vertic Paleoxeralf mi nicores using solutions of SAR 20 and EC 0.1 dS m(-1). The mellowing r atio was 0.2 when a solution of SAR 5 and EC 4 dS m(-1) was used. In s oils where aggregates collapsed without hierarchical breakdown, mellow ing ratios were always higher, indicating structural degradation.