## 2 Materials and Methods
### Research in Xunyi County, Shaanxi Province
The selenium content in the surface soils of the Guanzhong area of Shaanxi Province is 0.034-2.628 \(\times\) 10\({}^{-6}\), with an average content of 0.174 \(\times\) 10\({}^{-6}\)[21]. The enrichment coefficient is 0.75, and it still belongs to a low-selenium county in the loess area. [PERSON] et al. (2014) divided China into four regions based on soil selenium content. Regions with a selenium content \(\leq\) 0.1 mg/kg are selenium-deficient areas, those with selenium content of 0.1-0.2 mg/kg are low-selenium areas, those with a selenium content of 0.2-0.4 mg/kg are areas with normal selenium content, and those with a selenium content > 0.4 mg/kg are high-selenium areas [22]. All the soils in the whole Xunyi County area are alkaline and have less organic matter, and there is a general lack of selenium. \(\lx@sectionsign\)2 Selenium characteristics of different soil types  
According to the soil-type map of Xunyi County, the main soil types in this area are loessial soil, cinnamon soil, and black loam, and the elemental characteristics of the three soil types are shown in Table 2. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & **Sample Size** & **Mean** & **Median** & **Maximum** & **Minimum** \\ \hline As (10\({}^{-6}\)) & 427 & 13.07 & 13.35 & 15.80 & 6.65 \\ CaO (10\({}^{-2}\)) & 430 & 6.58 & 6.60 & 14.50 & 1.70 \\ Cd (10\({}^{-9}\)) & 429 & 174.33 & 170.00 & 230.00 & 118.84 \\ Cog (10\({}^{-2}\)) & 410 & 0.98 & 0.82 & 3.84 & 0.36 \\ Cr (10\({}^{-6}\)) & 427 & 69.04 & 69.10 & 84.20 & 48.90 \\ Cu (10\({}^{-6}\)) & 422 & 24.60 & 24.67 & 32.77 & 16.20 \\ Hg (10\({}^{-9}\)) & 405 & 28.77 & 27.13 & 229.00 & 12.20 \\ MgO (10\({}^{-2}\)) & 413 & 2.31 & 2.29 & 3.19 & 1.73 \\ Mn (10\({}^{-6}\)) & 423 & 652.9 & 650.0 & 887.37 & 432.0 \\ Mo (10\({}^{-6}\)) & 427 & 0.77 & 0.77 & 0.96 & 0.58 \\ Pb (10\({}^{-6}\)) & 431 & 22.41 & 22.40 & 26.60 & 18.20 \\ PH & 428 & 8.23 & 8.26 & 8.56 & 7.28 \\ Se (10\({}^{-6}\)) & 420 & 0.13 & 0.13 & 0.30 & 0.07 \\ Zn (10\({}^{-6}\)) & 422 & 68.51 & 68.25 & 96.20 & 43.50 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of soils in Xunyi County. \begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline Element & As (10\({}^{-6}\)) & CaO (10\({}^{-2}\)) & Cd (10\({}^{-9}\)) & Cog(10\({}^{-2}\)) & Cr (10\({}^{-6}\)) & Cu (10\({}^{-6}\)) & Hg (10\({}^{-9}\)) \\ \hline Cinnamon soil & 12.77 & 6.75 & 172.3 & 1.18 & 69.16 & 24.36 & 29.21 \\ Black loam & 13.93 & 6.37 & 189.2 & 0.74 & 70.78 & 25.78 & 33.74 \\ Loessial soil & 13.21 & 6.82 & 174.0 & 0.87 & 69.25 & 24.52 & 27.96 \\ \hline Element & Mn (10\({}^{-6}\)) & Mo (10\({}^{-6}\)) & Pb (10\({}^{-6}\)) & PH & Se (10\({}^{-6}\)) & Zn (10\({}^{-6}\)) & MgO (10\({}^{-2}\)) \\ \hline Cinnamon soil & 650.8 & 0.75 & 22.23 & 8.23 & 0.13 & 67.73 & 2.33 \\ Black loam & 670.3 & 0.79 & 22.94 & 8.2 & 0.14 & 69.47 & 2.24 \\ Loessial soil & 647.3 & 0.77 & 22.31 & 8.24 & 0.13 & 68.23 & 2.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Element characteristics of main soils in Xunyi. From the table, it can be seen that Se has similar contents in the three soils and is all in the low-selenium zone. It is known that Keshan disease mainly occurs in black loam. Although Se is relatively higher than that in loessial soil and cinnamon soil, it is still selenium-deficient. Using SPSS software (IBM spss statistics20) to calculate the correlation of elements and draw diagrams of the correlation between each element and selenium, as shown in Figure 1.  
It can be seen from the figure that the distribution of selenium in the soil of this area is positively correlated with Corg, Cd, Mn, Hg, Pb, etc. The most correlated is organic carbon. Due to the low level of soil organic matter, the effectiveness of Se is positively correlated with organic matter [23]. At the same time, Cd, Hg, Pb, etc., have an antagonistic effect on selenium, which can also affect the effectiveness of selenium. According to previous studies, the pH value range that is most suitable for selenium absorption is about 6.7-7.9 [24].