## 3 Results and Discussion
### Compost Characteristics
#### 3.1.1 Physicochemical Parameters
Previous surveys demonstrated large variations in pH values (lower than 9) of GW compost and that made from GW plus other biodegradable wastes: pH = 8.2-8.9 [35, 62], pH = 3.88-8.97 [25], pH = 4.5-6.9 [59], pH = 6.17-7.96 [21, 22, 32, 33, 63, 64, 65]. Some authors [57] considered that the alkaline pH of the substrate was the best for composting, while others stated that the high compost pH was not desirable, because if it exceeded 7.5-8.5, the nitrogen would be converted to NH\({}_{3}\) and evaporated as a gas emission [66, 67]. Thus, nitrogen transformation reduced the nitrogen reserve of the compost and worsened its properties as a fertilizer. Some researchers reported that the pH of good-quality compost should be below 9 [5, 68]; others claimed that pH values from 6.0 to 7.8 were needed for high-quality compost [69]. The pH of the compost may affect the soil quality. The use of compost with pH > 7 as soil fertilizer was not recommended, because it might cause losses of some microelements, i.e., Fe and Mn from soil, that are important for the plants [70]. Thus, the pH values of the produced GW composts could be defined as comparatively high and undesirable. _Electrical conductivity (EC)_ gives information about the amount of the dissolved salts in aqueous substrate and is an indirect indicator to assess the complex nutrient content in the compost (sodium, chloride, potassium, nitrate, sulfate and ammonium salts); therefore, it is an important parameter for the compost quality assessment. In the present study, the EC values were lower than 2.57 mS cm\({}^{-1}\) and varied for both tested composts. The average EC value of C2 was 1.48 times higher than that of C1 (\(p<0.05\)), while the coefficients of variation were close and revealed a slight variability of that parameter. The EC difference between the two composts could be attributed to their different moisture contents--21.8% (C1) vs. 17.9% (C2)--and substrate composition. The higher moisture of C1 (over 1.22 times compared to that of C2) determined better dilution of dissolved salts in C1 (and lower EC, respectively) than in C2. The different substrate composition of the tested composts suggested a different mineral content, but since it has not been studied, could be only hypothesized. Results similar to ours were reported in previous studies about GW compost (0.94-2.6 mS cm\({}^{-1}\)[21, 28, 32, 33, 62, 63]); as well as higher values, (6.73-9.07 mS cm\({}^{-1}\)[5]). Lower values were established for GW compost (0.33-0.51 mS cm\({}^{-1}\)[35]); and for compost from GW (48%), food waste (39%) and sawdust (13%), which were 0.37-0.51 mS cm\({}^{-1}\)[10]. Therefore, it can be assumed that the conductivity of the tested GW composts is usual for this type of compost. _Total Organic Carbon (TOC)_ concentration is an indicator of organic matter amount of the compost. TOC values varied between both produced composts--8.03% (C1) vs. 13.3% (C2)--but difference was not statistically significant (\(p>0.05\)). Coefficients of variation characterized TOC as a moderately variable indicator (Cv = 13.8-17.6%). TOC concentrations of monitored GW composts were partially comparable to the TOC concentration of compost, produced from leaves and grass (12.35% [65]); and lower concentrations were reported for GW compost and for compost from GW and food waste (18.4-24.76% [5, 10, 21, 22, 31]). It is noteworthy that the TOC content of various GW used for composting varied widely (21.60-56.31%), with values significantly exceeding those of the present study, as well as with higher coefficient of variation (Cv = 24.4% [18]). Obviously, the factors and conditions determining the organic carbon content of GW are multiple and different; that is why some types of GW contain more organic carbon and others less. Since the composts evaluated in the current study contained relatively low levels of organic carbon, it could be concluded that the raw materials used for their production, especially municipal green waste, were also poor in this element.