Influence of potassium salt on cracking behavior of purple soil under wetting–drying cycles

Abstract

The cracking behavior of purple soil during wetting-drying cycles is a common natural phenomenon in subtropical monsoon regions, posing risks of soil erosion and geological hazards such as landslides. The addition of salts changes the physicochemical properties of purple soil, which has a significant influence on the crack dynamics. Traditionally, NaCl is employed as a representative salt to investigate its influence on crack development during drying. However, NaCl is not the only prevalent salt in natural environments. In the humid and rainy, Sichuan Basin, where fertile purple soil predominates, potassium-based fertilizers, unlike NaCl, are widespread. Therefore, this study explores the effect of potassium salts (i.e., KCl, K₂SO₄, and plant ash) on cracking behavior of purple soil subjected to wetting–drying cycles. To achieve this, a combination of laboratory experiments, long-term monitoring, advanced image analysis techniques, and microscopic characterization is employed. The results indicate that KCl reduces crack formation, while K₂SO₄ enhances it compared to the salt-free sample after the first wetting-drying cycle. A small amount of plant ash (0.3 %–1.0 %) promotes cracking, whereas a higher content (2.0 %–3.0 %) suppresses it. As the number of wetting-drying cycles increases from one to twelve, the total crack length in most samples initially increases, peaking at 336–476 cm, before decreasing to 262–391 cm. In contrast, in samples with 2.0 %-3.0 % KCl, it decreases directly from 125 to 292 cm to 4–223 cm, but may temporarily increase after several cycles. The type and content of potassium salts exert a significant influence on cracking behavior, primarily governed by ion exchange reactions in montmorillonite, alterations in matric suction, shifts in mineral composition, and salt weathering.