## 3 Results and Discussion
### Compost Characteristics
#### 3.1.2 Macronutrients  
Four elements of that group were assayed: total KN; P and K; and Mg. _Nitrogen_ is one of the most important elements for plant growth. The results show that the TKN of C2 was 1.32 times higher compared to that of C1, although not significantly different (\(p>0.05\)).  
The coefficients of variation were very close and indicated a moderate variation of that parameter (Cv = 17.8-18.9%). Previously reported results for TN content in different GW composts were similar (1.37-1.76% [8; 10; 24; 35]), higher (2.0-3.25% [5; 21; 31; 32; 33]) or lower (0.32-1.01% [9; 63; 64; 65]) compared to the present data. Summary data from 33 studies indicated that various types of GW used for composting contained nitrogen within a larger range than the monitored composts (0.41-3.40%) and with a considerably higher coefficient of variation- Cv = 48.1% [18]. It is well-acknowleded that the total N content of compost can vary substantially based on feedstocks used, processing conditions, curing and storage [71]. The nitrogen amount largely determines the compost quality [23], as 10 to 21-25% of compost N content can be absorbed by plants in the first year of its soil applications [68; 72].  
_Phosphorus_ is a constituent of the complex nucleic acid structure of plants, which regulates protein synthesis and therefore is important for plants' cell division, generation of new tissue and complex energy transformations [73]. The TP contents of the two tested composts were relatively high, but different: C2 contained 1.68 times more TP than C1 (\(p<0.05\)), while the coefficients of variation were comparable (Cv = 7.24-9.75%) and characterized the indicator as slightly to moderately variable. These results are in line with previous data reported for GW compost (0.27-0.75% [5; 10; 21; 31; 32; 33]) but higher (in some cases, significantly higher) in comparison to the results of other research studies on GW compost, compost from GW and food waste, and GW for composting (0.04-0.38% [9; 18; 22; 24; 26]). In addition, the TP concentration of GW used for composting is reported as a parameter of large variability (Cv = 61.4%) [18]. Taking into account that high concentrations of extractable and potentially bioavailable plant phosphorus (30-40%) are typical for the green compost because of insignificant P-sorption capacity of organic matter [28], it can be concluded that the composts produced contained satisfactory levels of that nutrient.  
_Potassium_ is an element necessary for proper plant growth. Unlike the total P, Ca and Mg, essentially all of compost TK is plant-available [71]; that is why its amount in the compost is of great importance. TK concentrations varied within a broad range between the two tested composts (0.89-1.80%, \(p<0.001\)) with low coefficients of variation (Cv = 4.44-5.61%), determining slight variability of the parameter. The comparative analysis showed both partial agreement with earlier results (0.57-1.56% [26]), lower (0.22-0.76% [9; 21; 22; 24; 31; 32; 33]) or higher (2.92-3.37% [5]) TK concentrations in GW compost. Finally, it can be assumed that the results obtained for TK were within the usual limits for GW compost.  
_Mg_ belongs to the group of secondary nutrients (N, P and K are in the first group), but nevertheless, this essential macronutrient is also critical for plant growth and health. Similarly to the concentrations of the other macronutrients (N, P, K), Mg concentrations differed between the two tested composts and were 1.27 times higher (\(p>0.01\)) in C2 than in C1 (0.67% vs. 0.53%, respectively). The coefficients of variation were low and very close defining this parameter as slightly variable. The previously measured Mg levels were within a similar range (0.13-0.76% [9]), higher (0.93-1.19% [21; 32]) or lower (0.091% [65]) for GW compost. Therefore, the obtained results can be accepted as positioned in the middle range of analyzed data for Mg in GW compost.