## 5 Conclusions  
In this paper, Kenli County in the Yellow River Delta was selected as the research area. Twelve sample sets consisting of 908, 797, 700, 594, 499, 398, 299, 200, 149, 100, 75 and 50 sampling points were selected to study the influence of sampling number on the spatial variation of soil pH. A reasonable sampling number was determined, which provides for the collection of soil samples with minimum human, material and financial resources for research on soil pH.  
With the decreasing the sampling points number, the C0/Sill value of the sample set increased, and the spatial autocorrelation decreased gradually. The variation range of soil pH fitted by different numbers of points was greater than 4 km, and the spatial autocorrelation distance was relatively large. When the number of sampling points was reduced to 100, the range increased significantly, reaching 26.52, which was about 5 times that of 200 sampling points. Comprehensive analysis of ME, RMSE and ASE showed that when the number of sampling points was 150, prediction accuracy was the highest, which can satisfy the spatial variation expression of soil pH in the Yellow River Delta region.  
The pH value in Kenli County was generally between 7.8 and 8.1, and the soil was alkaline. Areas with high soil pH value were distributed in the middle and southwest of the study area. With a decreased number of sampling points, the detailed characteristics of spatial variation of soil pH gradually disappear. When the number of sampling points was 150, it could not only describe the spatial distribution of soil pH in detail, but also accurately describe the spatial distribution pattern of soil pH.  
Therefore, considering prediction accuracy, spatial distribution and research funding, it is suggested that the reasonable sampling number should be no less than 150 in the Yellow River Delta region, which is equivalent to at least 107 sampling points for 1000 km\({}^{2}\)The results of this study are applicable to areas with environmental and soil conditions similar to those in the Yellow River Delta and have reference significance for these areas. However, the number of sampling points will be different in other areas to reasonably express the spatial distribution of soil pH, and needs to be analyzed in combination with local environmental conditions.  
Conceptualization, Y.Z. and X.Z.; data curation, Y.Z. and M.D.; investigation, S.L. and X.Z.; methodology, M.D.; project administration, X.Z.; software, M.D.; supervision, S.L., X.Z. and X.S.; validation, M.D.; visualization, Y.Z.; writing-original draft, Y.Z.; writing-review & editing, X.Z., X.S. and D.C.; funding acquisition, X.S. and D.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key Research and Development Program of China (grant number 2021 YFD1900900), the Key Research and Development Program of Shandong Province, China (grant number 2021 CXGC010801, 2021 CXGC010804), Shandong Province Modern Agricultural Industry Technology System Cotton Post Innovation Team (grant number SDAIT-03-06), and the Talent Fund of Qingdao Agricultural University, China (grant number 1114344).  
The authors declare no conflict of interest.