Adverse effects of Ca2+ on soil structure in specific cation environments impacting macropore-crack transformation

The traditional view of Na+ as harmful and Ca2+ as beneficial doesn't always apply in multi-cationic soil solutions. Initially, adding Ca2+ promotes Na+ leaching, reducing salinity, but excess Ca2+ becomes counterproductive. As Na+ leaches, the soil's Ca2+-Na+-Mg2+ mix shifts to Ca2+-K2+-Mg2+, Ca2+'s function changes, even causing the opposite effect. To investigate the complex mechanism of Ca2+ to Na+-Mg2+ and K+-Mg2+, we conducted an indoor soil column experiment using saline water (4 dS m−1) with different cation compositions [Na+-Ca2+-Mg2+ (NCM), Na+-Mg2+ (NM), K+-Ca2+-Mg2+ (KCM), K+-Mg2+ (KM)] and deionized water as the control (CK). The results showed that NM exhibited the highest crack volume, while KM had the greatest macropore volume, with NM having approximately 15 % more crack volume than KM. Notably, only NM displayed a more pronounced inclination towards pore anisotropy value of 0 when compared to CK. NCM and KCM had higher pore anisotropy values than NM and KM. KM and KCM had more cracks angled ranging from 45–90° than NM and NCM. KCM notably decreased transitional macropores < 5.5 mm in length compared to KM, with no significant difference (P > 0.05) observed in widths < 2.5 mm between KCM and KM. NM displayed the shallowest macropore distribution and the highest variability in macropore length among all treatments. Only NCM showed significantly reduced variability in both macropore length and width compared to CK. In summary, Ca2+ exhibited distinct action patterns on K+-Mg2+ and Na+-Mg2+. For specific soil types and cationic compositions, Ca2+ may not fully exert its amendment effects. However, Ca2+'s effect is soil-specific, necessitating comprehensive studies across varied soil types.