## 4 Discussion
The least sensitive indicator for contamination with tellurium oxide and nitrate was the _Azotobacter_ sp. abundance. When contaminated with oxides of silver, bismuth, tellurium, and thallium, the most sensitive biological indicator was the length of wheat roots; when contaminated with nitrates, the most sensitive biological indicator was the total number of bacteria. The least sensitive biological indicator for contamination with both chemical forms of silver, bismuth, tellurium, and thallium was the _Azotobacter_ sp. abundance. The information content was evaluated by Spearman correlation coefficients (R) between the content of heavy metals and the response of biological indicators for 10, 30, and 90 days (Table 3). It was established that for contamination with silver, the series of informative biological indicators looked as follows:  
oxide of silver: Az > A\({}_{\rm{deh}}\) > A\({}_{\rm{cat}}\) >LR > B > G nitrate of silver: Az > A\({}_{\rm{deh}}\) = LR > G > A\({}_{\rm{cat}}\) > B  
The most informative biological indicator for contamination with silver oxide and nitrate was the _Azotobacter_ sp. abundance. The least informative indicator for contamination with silver oxide was germination; for nitrate, it was the total number of bacteria. It was established that for contamination with bismuth, the series of informative biological indicators looked as follows:  
oxide of bismuth: Az > A\({}_{\rm{cat}}\) > G > A\({}_{\rm{deh}}\) > LR > B nitrate of bismuth: Az > G > A\({}_{\rm{deh}}\) > A\({}_{\rm{cat}}\) = LR > B \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**R**} & \multicolumn{2}{c}{**Chemical**} & \multicolumn{5}{c}{**Biological Indicators**} \\ \cline{2-9}  & \multicolumn{2}{c}{**Form**} & & & & & & \\ \hline \multirow{2}{*}{Silver} & \multicolumn{2}{c}{\(\mathbf{A_{\rm{cat}}}\)} & \(\mathbf{A_{\rm{deh}}}\) & \(\mathbf{Az}\) & \(\mathbf{B}\) & \(\mathbf{G}\) & \(\mathbf{LR}\) \\ \cline{2-9}  & oxide & \(-\)0.86 & \(-\)0.96 & \(-\)0.99 & \(-\)0.66 & \(-\)0.61 & \(-\)0.76 \\ \cline{2-9}  & nitrate & \(-\)0.72 & \(-\)0.90 & \(-\)0.96 & \(-\)0.71 & \(-\)0.88 & \(-\)0.91 \\ \hline \multirow{2}{*}{Bismuth} & oxide & \(-\)0.90 & \(-\)0.72 & \(-\)0.96 & \(-\)0.63 & \(-\)0.79 & \(-\)0.66 \\ \cline{2-9}  & nitrate & \(-\)0.81 & \(-\)0.83 & \(-\)0.95 & \(-\)0.78 & \(-\)0.86 & \(-\)0.81 \\ \hline \multirow{2}{*}{Thallium} & oxide & \(-\)0.75 & \(-\)0.74 & \(-\)0.99 & \(-\)0.81 & \(-\)0.86 & \(-\)0.89 \\ \cline{2-9}  & nitrate & \(-\)0.96 & \(-\)0.82 & \(-\)0.99 & \(-\)0.52 & \(-\)0.79 & \(-\)0.79 \\ \hline \multirow{2}{*}{Tellurium} & oxide & \(-\)0.84 & \(-\)0.93 & \(-\)0.99 & \(-\)0.80 & \(-\)0.73 & \(-\)0.74 \\ \cline{2-9}  & nitrate & \(-\)0.90 & \(-\)0.74 & \(-\)0.99 & \(-\)0.58 & \(-\)0.77 & \(-\)0.74 \\ \hline \end{tabular} Note: A\({}_{\rm{cat}}\)-activity of catalase; A\({}_{\rm{deh}}\)-activity of dehydrogenases; Az–_Azotobacter_ sp. abundance; B–number of soil bacteria; G—_germination; LR—_length of roots.  
\end{table} 
Table 3: Informativeness (Spearman’s correlation) of biological indicators after contamination by silver, bismuth, tellurium, and thallium. The most informative biological indicator for contamination with bismuth oxide and nitrate was the _Azotobacter_ sp. abundance. The least informative indicator for contamination with bismuth oxide, and for nitrate it was the total number of bacteria. It was found that when contaminated with thallium, the sensitivity of biological indicators ranged as follows:  
oxide of thallium: Az > LR> G > B > A\({}_{\text{Cat}}\) =A\({}_{\text{deh}}\)  
nitrate of thallium: Az > A\({}_{\text{Cat}}\) >A\({}_{\text{deh}}\) > G = LR > B  
The most informative biological indicator for contamination with thallium oxide and nitrate was the _Azotobacter_ sp. abundance. The least informative indicator for thallium oxide contamination was the activity of catalase and dehydrogenase; for nitrate of thallium, the least informative indicator was the total number of bacteria.