## 2 Materials and Methods
### Research in Xunyi County, Shaanxi Province
The soil in this area is 7.28-8.56, in the middle of the most suitable pH range. The closer the soil pH is to 7.28, the higher the total amount of Se, and the more the soil tends to be neutral, which further affects the absorption of selenium. Characteristics of selenium in the high-incidence area of Keshan disease. In order to further study the selenium content in the soil and crops in high-incidence areas, soil samples and crops were collected in high-incidence areas such as Dongcao Village, Tuqiao Town, Zhichian Town, and Changshetou Village, for a total of 25 sets of wheat and root soil, 2 soil profiles, etc. Characteristics of selenium in soil and crops in the high-incidence area. The characteristics of wheat samples, the main crop in the area, are shown in the following Table 3.  
It can be seen that the mean value of selenium in the soil of the high-incidence area is 0.12 \(\times\) 10\({}^{-6}\), slightly lower than the average value of soil in the whole county, which is  
Figure 1: Correlation of each element with selenium. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & Pb (10\({}^{-6}\)) & Zn (10\({}^{-6}\)) & Cr (10\({}^{-6}\)) & Mo (10\({}^{-6}\)) & Ge (10\({}^{-6}\)) & Se (10\({}^{-6}\)) \\ \hline \multirow{3}{*}{High-incidence area soil} & 25.41 & 80.71 & 68.64 & 0.89 & 1.05 & 0.12 \\ \cline{2-7}  & Corg (10\({}^{-2}\)) & pH & Mn (10\({}^{-6}\)) & Ca (10\({}^{-2}\)) & Fe (10\({}^{-2}\)) & \\ \cline{2-7}  & 1.7 & 8.44 & 0.07 & 3.68 & 3.32 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Element content in root soil in high-incidence areas. 0.13 \(\times\) 10\({}^{-6}\). Compared with the Guanzhong area [25], the mean value of selenium in the soil of the Guanzhong Plain is 0.171 \(\times\) 10\({}^{-6}\), and the selenium in the soil of the high-incidence area is significantly lower, with an enrichment coefficient of about 0.702,as shown in Table 4.  
The mean value of wheat in the high-incidence area is 0.014 \(\times\) 10\({}^{-6}\). Referring to the relevant selenium content standards for selenium-rich and selenium-containing soil in Shaanxi Province, it is the selenium-containing standard. Compared with the Guanzhong area [26], the mean value of selenium in the wheat of the Guanzhong Plain is 0.056 \(\times\) 10\({}^{-6}\), and the enrichment coefficient is 0.25. 2 Selenium availability in root zone soil  
The existing forms of selenium in the soil determine the ability of crops to absorb this element [27]. Existing studies have shown that the forms of selenium in the soil can be roughly divided into water-soluble, ion-exchangeable, carbonate, humic acid, iron-manganese oxide-bound, organic matter-bound, and residual states [28; 29]. Based on bioavailability considerations, water-soluble and ionic states are classified as available states; carbonate-bound and iron-manganese oxide-bound Se are the direct source of available Se and can be classified as potentially available states; while humic acid-bound, organic-bound, and residual-state Se can be classified as non-available Se. There are significant differences in the valence content of selenium in the soil of this area, with most being in the residual and strong organic forms, as shown in Figure 2.  
It can be seen from Figure 3 that the forms of selenium in the area are in the order of residual \(>\) strong organic \(>\) humic acid \(>\) water-soluble \(>\) carbonate \(>\) iron-manganese binding \(>\) ion-exchangeable. By statistically analyzing the content of available Se, potentially available Se, and non-available Se in the area, it is found that Se in the area is mainly in the non-available state, accounting for 84.81% of the total Se, the available Se is only 8.75%, and the potentially available Se accounts for 6.45% of the total Se. Compared with the soil selenium in the tillage layer of soil in Zhejiang, Qinghai, and Jilin, the proportion of available Se is the same, which is higher than that of red soil and black soil [30; 31; 32].