## 3 Results and Discussion
### Inorganic Substances (Metals)
Therefore, one should respect the legislation's threshold concentrations, as Zn, along with other metals, may be toxic to the soil-plant system and in inadequate concentrations, inhibits the development of seeds, fruits, and roots [61]. Various studies consider the bioaccumulation by metals insignificant when biosolids are applied in agricultural soil again and again [62; 63]. Ref. [63] documented that 16 years in a row of applications of biosolids in agricultural soils, in Spain, did not cause toxic effects on the soil-plant system. Ref. [62] documented that three years of application of biosolids did not significantly alter the concentrations of metals in the soil. Conversely, Ref. [64], in a study where biosolids were applied in 31 areas for more than 100 years without audit or control of the application rates, documented that the total concentrations of metals in the soil were a common source of contamination in all the areas. The authors reported that Zn was the most abundant element (122-2050 mg kg\({}^{-1}\)), Cu varied from 25.3 to 766 mg kg\({}^{-1}\), Cd from 0.43 to 48.6 mg kg\({}^{-1}\), Cr from 43.2 to 1670 mg kg\({}^{-1}\), and Pb from 68.6 to 688 mg kg\({}^{-1}\). However, the results of the previously cited study may and should be contested since no control of the application rate is conducted over time. Ref. [65] conducted an application of biosolids for ten years in Chinese soils and reported that the biosolids in their study could be applied safely to the soil at an application rate of 7.5 (tons ha\({}^{-1}\)) for 18 years without exceeding the limits for Hg. As for Zn, the biosolids could be applied for 51 years, and more than 51 years for other heavy metals. In addition, the content of heavy metals in wheat and corn grains met the Chinese legislation's hygienic standard, and the Zn content in wheat straws and corn linearly increased with the increase in the biosolids application rates. Ref. [66] aimed to study three years of application of biosolid treated with and without lime for forage fertilization aimed at animal consumption and concluded that in relation to the Cu and Zn metals the quality of forage plants was adequate for consumption. In a similar context, it is known that biosolids have a concentration of metals lower than that in diverse organic residues, for example, manure of cattle, chickens, sheep, and pigs [15]. Ref. [67] investigated the concentration of metals in bovine manure, broiler chickens, laying hens, sheep, and swine for slaughter, and found out that the As concentration in those fertilizers was higher than the average concentration found in our study of 0.49 \(\pm\) 1.77 (mg As kg\({}^{-1}\)), as per Table 2. The 0.04 \(\pm\) 0.14 (mg Cd kg\({}^{-1}\)) Cd concentration met the limits of the three norms, as well as was lower than that found in swine manure and other organic manures, such as manure of beef and dairy cattle and laying hens, of the study of [67]. Regarding Cu, with an average concentration of 92.45 \(\pm\) 84.49 (mg Cu kg \({}^{-1}\)), in addition to meeting the legislation limits, it was lower than the one found by [67; 68] in pig manure. Ref. [69] documented that applications of chicken and swine manure over three years significantly reduced the accumulation of Pb and Cd in rice grains, and decreased the Cd and Pb available in the soil. The values found in the authors' organic residues were 6.90 (mg Pb kg\({}^{-1}\)) and 0.41 (mg Cd kg\({}^{-1}\)) for chicken manure, and 9.20 (mg Pb kg\({}^{-1}\)) and 0.54 (mg Cd kg\({}^{-1}\)) for pig manure, which are higher than the average concentrations found in the biosolids of the present study (Table 2).