## 4 Discussion
It was found that when contaminated with tellurium, the sensitivity of biological indicators ranged as follows:  
oxide of tellurium: Az >A\({}_{\text{deh}}\) > A\({}_{\text{cat}}\) > B > G = LR  
nitrate of tellurium: Az > A\({}_{\text{cat}}\) > G >A\({}_{\text{deh}}\) = LR > B  
The most informative biological indicator for contamination with tellurium oxide and nitrate was the _Azotobacter_ sp. abundance. The least informative indicator for contamination with tellurium oxide was the _Azotobacter_ sp. abundance. The least informative indicator for contamination with tellurium oxide was root length and germination, and for nitrate of tellurium it was the total number of bacteria. The most informative biological indicator for contamination with both chemical forms of silver, bismuth, tellurium, and thallium was the _Azotobacter_ sp. abundance. Germination was the least informative for silver oxide; for bismuth, the total number of bacteria was the least informative; for thallium oxide, it was the activity of catalase and dehydrogenases; and for tellurium oxide, it was the root length and germination. The least informative biological indicator for contamination with silver, bismuth, tellurium, and thallium nitrates was the total number of bacteria. Evaluation of a wide range of enzymes of soil when contaminated with both chemical forms of silver, bismuth, tellurium, and thallium allowed establishing that catalase and phosphatase had the greatest sensitivity, and the most informative enzymes were phosphatase and invertase [44]. According to the ecotoxic effect of heavy metals on the state of the soil after 90 days of the experiment after contamination, a series is presented, % of the pure soil:  
oxides: bismuth (82) < silver (78) = tellurium (78) < thallium (77)  
nitrates: silver (80) = bismuth (80) < thallium (60) < tellurium (56)  
The most toxic elements among those studied, both in the form of oxides and in the form of nitrates, were thallium and tellurium. According to the integral indicator of soil enzymatic activity (IIEA), thallium contamination was detected at a critical maximum permissible concentration of 0.601 mg/kg, which was two times higher than silver (0.396 mg/kg), but three times less than tellurium (1.475 mg/kg) [44]. Previously, it was found that tellurium (Te), selenium (Se), silver (Ag), and chromium (Cr) had the greatest toxic effect on the biological functions of the soil [5]. Thus, when tellurium and thallium nitrates were introduced into the soil, the maximum decrease of the biological parameters of chernozem was established. Enzymatic activity, number of bacteria, phytotoxicity of such rare elements as silver, bismuth, tellurium, and thallium have been studied to a different extent. If the change in activity during pollution with silver and bismuth compounds has been studied to some extent [10; 22; 23; 37; 39; 41; 65], then with pollution with thallium and tellurium, these are isolated works [38; 42]. This contamination by AgNPs in the total number of bacteria and enzymatic activity was more than that in the _Azotobacter_ sp. abundance [38]. The highest sensitivity for contamination with bismuth was found in terms of the number of bacteria (61%) and the activity of dehydrogenases (77%), and the highest information content in terms of _Azotobacter_ sp. abundance (R = \(-\)0.55) and the germination rate (R = \(-\)0.68) [41]. In our study, pollution with silver and bismuth has the greatest effect on the sensitivity of the length of the roots and the number of bacteria, and _Azotobacter_ sp. abundance is the most informative. When Haplic Chernozem is contaminated with thallium, the most sensitive indicators are the length of the roots and the number of bacteria, and for tellurium, the most sensitive indicator is germination.