Non-Synergistic Changes in Migration Processes between Soil Salt and Water in the Salt Patch of the Coastal Saline Soil

Salt patches (SPs) with surface salt accumulation pose a serious threat to agriculture in coastal saline lands. However, the migration and distribution of soil water and salt in SPs remain unclear due to complex water–salt transport dynamics. In this study, we focused on typical SPs in the Yellow River Delta region and selected center site (Site 1), transition site (Site 2), edge site (Site 3), and outer site (Site 4) with varying levels of salinization. Field sampling and the HYDRUS-1D model were employed to investigate the migration process and distribution of soil water and salt in SPs, as well as the influencing factors. The results indicated significantly higher salt contents in the central sites (Site 1 and Site 2) compared to the edge sites (Site 3 and Site 4), while no significant differences were observed in soil water content. The bottom soil exhibited greater stability in terms of water and salt content compared to the surface soil. Additionally, soil water content increased with soil depth, whereas salt content decreased from Site 1 to Site 3. Interestingly, Site 4 exhibited the opposite salt distribution pattern in the whole soil depth. We observed that SPs displayed a salt aggregation structure radiating from the center to the periphery, gradually weakening in intensity. Our correlation analysis indicated that the formation of SPs may be influenced by soil particle size distribution, precipitation, and evaporation. Specifically, fine soil structure can impede the upward transport of highly mineralized groundwater, while precipitation and evaporation directly affect the leaching and upward movement of surface soil salt, resulting in uneven salt distribution in the field and the formation of SPs. These findings provide valuable theoretical and technical insights for the prevention and improvement of saline farmlands in the Yellow River Delta.