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Soil Structure Degradation and Mellowing of Compacted Soils by Saline-Sodic Solutions

Dep. of Soil Science, College of Agriculture, Univ. of Ahwaz, Ahwaz, Iran

J. Malcolm Oades and Pichu Rengasamy

Dep. of Soil Science, Waite Agricultural Research Inst., Univ. of Adelaide, and Co-operative Research Centre for Soil and Land Management, Private Bag no. 1, Glen Osmond, SA 5064, Australia (rbarzega{at}waite.adelaide.edu.au)

ABSTRACT

Aggregates are formed by joining of structural units of different sizes in a hierarchical order. During wetting, the aggregates may either disintegrate completely (slaking) or remain intact with only loosening at the points of weakness (mellowing). This study investigated the effect 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 number 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-µm aggregates compared with the original soils. The values for the Vertic Palexeralf were three times those of the Typic Haploxeralf. In contrast, sodic solutions led to the collapse of aggregates. Mellowing ratios of aggregates changed after 10 wetting-drying cycles and were 0.6 to 0.7 for SAR 5 and 0.8 to 1 for SAR 20 for the Vertic Palexeralf; for the Typic Haploxeralf they were 0.7 to 0.8 and 0.9 to 1.2 for SARs 5 and 20, respectively. Mellowing ratios between 0.9 and 1 were obtained for the Vertic Paleoxeralf minicores using solutions of SAR 20 and EC 0.1 dS m^-1. The mellowing ratio was 0.2 when a solution of SAR 5 and EC 4 dS m^-1 was used. In soils where aggregates collapsed without hierarchical breakdown, mellowing ratios were always higher, indicating structural degradation.