## 2 Materials and Methods
### Research in Xunyi County, Shaanxi Province
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & Pb (10\({}^{-6}\)) & Zn (10\({}^{-6}\)) & Cr (10\({}^{-6}\)) & Mo (10\({}^{-6}\)) \\ \hline \multirow{3}{*}{Wheat in the high-incidence area} & 0.04 & 20.72 & 0.4 & 0.89 \\ \cline{2-5}  & Se (10\({}^{-6}\)) & Mn (10\({}^{-6}\)) & Ca (10\({}^{-2}\)) & Fe (10\({}^{-2}\)) \\ \cline{2-5}  & 0.014 & 47.8 & 426.83 & 36.86 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Characteristics of wheat in the high-incidence area. Figure 2: Proportion of selenium valence in soil. Based on the characteristics of root zone soil elements and the availability of soil selenium, the SPSS software (IBM spss statistics20) was used to calculate the correlation between availability and each element, as shown in the histogram of the correlation between availability and each element. It can be seen from the figure that the effectiveness of soil selenium is weakly positively correlated with the total amount of Se and organic matter. According to the research, when the overall content of soil organic matter is low, within a certain range, a higher content of soil organic matter will increase the content of organic selenium. Along with the mineralization of organic selenium, the small molecule organic selenium and inorganic selenium that are easily dissolved in the soil will increase, thereby improving the biological effectiveness of selenium [33]; the organic matter in the loess area is generally low, which makes the selenium effectiveness positively correlated with the organic matter, but when the organic matter rises to a certain extent, it can show a decrease in the effectiveness of Se. At the same time, the available selenium in the soil is negatively correlated with elements such as Zn, Mn, Mo, and Pb, reflecting that the lower the content of these metal elements in the soil, the higher the effectiveness of selenium. 4 Enrichment coefficient of wheat selenium  
In order to reflect the enrichment of Se in the soil-agricultural crops, the enrichment factor (EF) is expressed by EF = wheat Se/root soil Se. The maximum value of EF in this area is 0.22, the minimum value is 0.1, and the average value is 0.14. Based on the measurement results, the selenium coefficient of wheat and root soil was calculated. The correlation diagram of the enrichment coefficient of wheat and the various elements of the root soil is shown in Figure 4.  
Figure 4: Correlation between wheat Se enrichment coefficient and root soil elements. Figure 3: Correlation diagram of selenium effectiveness and various elements. It can be seen from the figure that most elements are positively correlated with the wheat selenium enrichment coefficient, among which Mo and Zn have the greatest influence coefficient. The acidity and alkalinity are negatively correlated with the wheat selenium enrichment coefficient. It reflects that in this area, the wheat-producing areas have a high Mo and Zn, and the pH tends to be neutral. The higher the enrichment coefficient, the higher the selenium content in wheat. 5 Vertical profile selenium characteristics  
Samples were collected from the profile PM01 in Dongcao Village in the high-incidence area, with a profile depth of 2.0 m and sampling every 20 cm; Samples were collected from the profile PM02 in Zhitian Town Zaochi Village, with a profile depth of 2.2 m and sampling every 20 cm. The results of the selenium and available selenium profile are shown in Figures 5 and 6. From the curves of the change in selenium and available selenium with depth in the two profiles, it can be concluded that the overall selenium in the soil in this area is relatively low.