Spatial Variability of Soil Salinization and Alkalization in the Northern Plain Irrigation Area of Yinchuan, Ningxia

Soil salinization is a type of soil degradation phenomenon caused by the gradual accumulation of salt in soil due to natural processes or improper irrigation practices. Understanding the spatial variations of salinization is the basis for salinized land management and resource utilization. Taking Yinbei Plain irrigation area of Ningxia as the research area, 154 sampling points were set up and soil samples were collected at depths from 0–100 cm in six layers to determine the pH value and total salt content of the soil. Then geostatistics and GIS were used to study the spatial variations of soil pH and total salt content in the irrigation area. The results show three important aspects of this system. (1) The soil in the study area is alkaline soil with pH values ranging from 8.18 to 10.22, and the pH increases with increasing soil depth. The pH values of soil at 0–10 cm are significantly higher than at depths of 10–20 cm and 20–80 cm (P<0.05). The variation coefficients of the soil pH value in each layer are less than 10%, which is characterized as weak variability. The total salt content of soil decreases with increasing soil depth, ranging from 0.24 to 14.95 g kg–1. The total salt content of soil in the surface layer at 0–10 cm was significantly higher than in the lower soil layers (P<0.05). The coefficient of variation of total salt content showed moderate variability except at depths of 10–20 cm and 80–100 cm. Other layers have variabilities of more than 100%, indicating strong variability. (2) The semi-variance function fitting of pH and total salt showed that the soil pH value in each soil layer depth block base ratio ranged from 28.1% to 61.2%, which indicates a medium spatial correlation, and the spatial variability was mainly influenced by the joint action of human activities and natural factors. The total salt block base ratio of each soil layer ranged from 5.5% to 13.3%, which indicates strong spatial autocorrelation. The variation among soil samples is mainly caused by natural structural factors such as parent material, topography, soil and groundwater level. (3) Kriging interpolation was used to obtain the spatial distribution of soil pH values and total salt contents in the study area, and the results showed that the overall pH distribution was high on the eastern and western sides and low in the middle. The total salt distribution was patchy and uneven, and the saline soil was mainly distributed in the surface layer. As the depth increased, the total salt content declined, and the proportions of saline soil and severely salinized soil decreased. In the whole study area, mild salinized soil was the main category, the total salt content increased from south to north, and the profile characteristics of total salt content were mainly surface clustering. This study investigated the distribution and spatial heterogeneity of soil salinization in the Yinbei Irrigation Area, and provides theoretical guidance for the formulation of soil salinization control measures in this area.