Cation composition of saline water affects soil structure by altering the formation of macropores and cracks in illite soils

Cation ratio of soil structural stability (CROSS) is preferred over sodium adsorption ratio (SAR) for assessing irrigation water quality. However, its applicability in evaluating the impact of high-CROSS saline water on soil cracking remains unknown, as previous studies have focused solely on clay particle dispersion and flocculation. This study aims to determine whether high-CROSS saline water, with identical EC values, induces soil cracking and to identify the CROSS thresholds that trigger it. An indoor soil column test was conducted to analyze soil clay dispersion, macropores, and crack alterations after 12 wet and dry cycles. Saline water with the same EC of 4 dS m−1, and different cation compositions (thus different CROSS levels) of Na+-Mg2+ (NM100), Na+-K+-Mg2+ (NKM67), Na+-Ca2+-Mg2+ (NCM38), Na+-Ca2+ (NC28) and K+-Ca2+ (KC9) (the figure of each treatment’ name represents the value of optimal CROSS (CROSSopt)), respectively. Results indicated that soil suspension turbidity increased with rising CROSS. When CROSS surpassed a specific threshold, it led to soil cracking, with crack angles transitioning from horizontal to vertical with increasing CROSS. Notably, NKM67 induced significant vertical crack formation. In NM100, intersecting vertical and horizontal cracks led to a drastic drop in macropore numbers, with narrow cracks evolving into those with smaller aspect ratios. While KC9's crack volume was about 5% of its macropore volume, crack volumes in NC28, NCM38, and NKM67 were nearly the same as their macropore volumes. However, NM100's crack volume was about 60 times its macropore volume. Independent pores remained below 20% of connected pores, with KC9 primarily comprising connected macropores, while NM100 had connected cracks. However, the results obtained in this study are soil-specific and might vary depending on soil texture and mineral types in other regions. More comparative studies on various soils are recommended for general use in the future. At all events, these findings provide a new inspiration for studying the safe utilization of saline water and preserving soil quality.