# On-Farm Composting of Hop Plant Green Waste--Chemical and Biological Value of Compost
## 3 Results and Discussion
### Chemical Characterization  
The pH of compost BC and EM increased significantly compared to the input material in our experiment (Tables 3 and 4). When we compare the pH of the final composts, we can see that there are significant differences among them (Table 4), with the highest pH in compost EM. Low pH in starting material can be assigned to organic acids' presence and high pH at the end of the process to the high content of NH\({}_{4}\)-N. The average pH of final compost (7.8) is within the required range of 6.0 to 8.5 for mature composts [31; 34; 35]. One of the factors of increasing pH in composts during composting could be the decomposition of nitrogen-containing organic matter leading to the accumulation of ammonia that dissolves in water fractions to form alkaline NH\({}_{4}\)\({}^{+}\)[36]. In addition, the combination of available K\({}^{+}\) in water-soluble form with bi-carbonic acids (HCO\({}_{3}\)\({}^{-}\)) produced during organic matter mineralization leading to the generation of potassium hydroxide could affect pH value [37]. In our study, at the beginning of composting, the pH value correlated with K content in input biomass. For instance, the pH of input material EM was the lowest, as well as the total K content was, in EM, the lowest compared to others. With the aging of composts, the accumulation of ammonia increased, which could cause an increase in compost pH. Nonetheless, composts should be used in accordance with the pH requirements of a certain plant [31]. Because the soil in the Lower Savinja Valley is slightly acidic [38], fertilization with slightly alkali compost could be beneficial. Dry matter (DM) content slightly increased from an average of 27.8 to 31.2% (Tables 3 and 4). According to [PERSON] [39], compost should have a DM content of 30-50%. The final compost BC with 28.6% did not reach this value. In comparison with the other two piles, the final compost CON had significantly higher DM (34.8%) (Table 4). Composts were exposed to weather conditions; therefore, the variation in DM at the end of composting was expected. Figure 2: Temperature dynamics during composting are represented by the lines (CON, BC, and EM), the shapes at the bottom of the graph represent compost turning dates. Bottom line represents ambient temperature (AT). The average total nitrogen (TN) content of starting material was 2.6% and differed significantly between the piles; the content in pile EM was much lower than in the other two piles (Table 3). The highest level of TN was observed in compost BC (3.4%), followed by compost EM (2.6%), and the lowest TN content in the final compost was found in pile CON (2.0%) (Figure 3). TN increased in compost piles BC and EM (both with additives) and diminished in compost CON during composting (Figure 4a). The reason for this can be attributed to added biochar, which reduces nitrogen losses from composting material [33; 40]. Biochar has been increasingly used as an adsorbent to reduce N loss during composting in recent decades [19; 20; 33]. In the laboratory experiment of [PERSON] et al. [41], the adsorption rate and adsorption capacity of biochar as an adsorbent in the purification of clear-cut forest runoff water were studied. Biochar decreased TN concentrations in runoff water. The adsorption of TN was detected in all biochar treatments. Analysed compots commonly contain around 2% nitrogen, 0.5-1% phosphorus and 2% potassium. As a general rule, compost and manure with a total nitrogen value greater than 2% can be used as a fertilizer, so all composts cover this requirement. Comparing the TN content in 1 tonne of fresh material (8 kg/t), average final compost contained a comparable amount as cattle manure compost [42].