| Summary: | As an important index reflecting the degree of soil salinization and alkalization, the proportion of saline-alkali patches (PSP) is beneficial for identifying the degree of saline-alkali land on a large scale with high spatial heterogeneity. However, determining the optimal scale for observing PSP remains inconclusive. In this article, we considered a typical area of saline-alkali land, which is located in Da'an City in Northeast China, as an example and selected nine sample areas from the sentinel-2A remote sensing images to calculate the salt index. According to the semivariogram function, we obtained the spatial autocorrelation range of the average radius of saline-alkali land. By combining the Nyquist-Shannon sampling theory, we determined the optimal scale for the remote sensing monitoring of saline-alkali land in Da'an City. The PSP index is calculated using this scale. The results were as follows: First, the optimal scale for the remote sensing monitoring of saline-alkali land in Da'an City is 41.2 m, and the calculated PSP index at this scale has the best correlation with the measured <italic>pH</italic> value data (<italic>R</italic><sup>2</sup> = 0.828). Second, Using the PSP index calculated at this scale, we divided the degree of saline-alkali land in Da'an City into three grades. Among them, the slightly, moderately, and severely saline-alkali lands cover areas of 90.52, 111.78, and 959.77 km2, respectively. In this article, we propose a new method for grading the degree of soil salinization and alkalization on land with high spatial heterogeneity, providing technical support for the utilization of saline-alkali land.
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