## 4 Discussion  
The use of biological indicators to assess the state of contaminated soils before assessing the quantitative content of a pollutant allows an indirect assessment of the resistance of soil microbiota to pollution. At the same time, it is very important to choose among the variety of indicators those that are the most informative (according to the tightness of the relationship with the concentration of the pollutant) and sensitive (according to the largest percentage of the indicator decrease relative to the pure soil) [4]. Sensitivity assessment was carried out based on the response of biological indicators to contamination with both chemical forms of silver, bismuth, tellurium, and thallium (Table 2). It was found that when contaminated with silver, the sensitivity of biological indicators ranged as follows:  
oxide of silver: LR = B > G > A\({}_{\text{deh}}\) > A\({}_{\text{cat}}\) = Az nitrate of silver: B > G = LR > A\({}_{\text{cat}}\) > A\({}_{\text{deh}}\) > Az  
The most sensitive biological indicator for contamination with silver oxide was root length, and for nitrate of silver it was the total number of soil bacteria. The least sensitive indicator for contamination with silver oxide and nitrate was the _Azotobacter_ sp. abundance. It was found that when contaminated with bismuth, the sensitivity of biological indicators ranged as follows:  
oxide of bismuth: LR > B = G > A\({}_{\text{deh}}\) > A\({}_{\text{cat}}\) > Az nitrate of bismuth: B > G = LR > A\({}_{\text{cat}}\) > A\({}_{\text{deh}}\) > Az  
The most sensitive biological indicator for contamination with bismuth oxide was root length, and for nitrate of bismuth it was the total number of soil bacteria. The least sensitive indicator for contamination with bismuth oxide and nitrate was the _Azotobacter_ sp. abundance. It was found that when contaminated with thallium, the sensitivity of biological indicators ranged as follows: \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{\
\begin{tabular}{c} **Heavy** \\ **Metals** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Chemical** \\ **Form** \\ \end{tabular} } & \multicolumn{6}{c}{**Biological Indicators**} \\ \cline{3-8}  & & **A\({}_{\text{cat}}\)** & **A\({}_{\text{deh}}\)** & **Az** & **B** & **G** & **LR** \\ \hline \multirow{3}{*}{Silver} & oxide & 90 & 86 & 90 & 66 & 74 & 65 \\ \cline{2-8}  & nitrate & 82 & 90 & 94 & 57 & 79 & 80 \\ \hline \multirow{3}{*}{Bismuth} & oxide & 90 & 89 & 98 & 76 & 75 & 66 \\ \cline{2-8}  & nitrate & 89 & 91 & 99 & 47 & 78 & 78 \\ \hline \multirow{3}{*}{Thallium} & oxide & 83 & 83 & 99 & 64 & 67 & 68 \\ \cline{2-8}  & nitrate & 71 & 62 & 86 & 40 & 58 & 44 \\ \hline \multirow{3}{*}{Tellurium} & oxide & 82 & 89 & 100 & 65 & 68 & 64 \\ \cline{2-8}  & nitrate & 64 & 52 & 93 & 32 & 56 & 38 \\ \hline \end{tabular} Note: A\({}_{\text{cat}}\)—activity of catalase; A\({}_{\text{deh}}\)—activity of dehydrogenases; Az–_Azotobacter_ sp. abundance; B—number of soil bacteria; G—permutation; LR—length of roots.  
\end{table} 
Table 2: Sensitivity of biological indicators of soil after contamination by silver, bismuth, tellurium, and thallium, % of pure soil (averaged PPC), % of pure soil. oxide of thallium: B > G > LR > A\({}_{\rm{deh}}\) = A\({}_{\rm{cat}}\) > Az nitrate of thallium: B > LR >G > A\({}_{\rm{deh}}\) > A\({}_{\rm{cat}}\) > Az  
The most sensitive biological indicator for contamination with thallium oxide and nitrate was wheat germination. The least sensitive indicator for contamination with thallium oxide and nitrate was the _Azotobacter_ sp. abundance. It was found that when contaminated with tellurium, the sensitivity of biological indicators ranged as follows:  
oxide of tellurium: LR = B > G > A\({}_{\rm{cat}}\) > A\({}_{\rm{deh}}\) > Az nitrate of tellurium: B > LR > A\({}_{\rm{deh}}\) > G > A\({}_{\rm{cat}}\) > Az  
The most sensitive biological indicator for contamination with tellurium oxide was root length, and for nitrate of tellurium it was the total number of soil bacteria.