# On-Farm Composting of Hop Plant Green Waste--Chemical and Biological Value of Compost
## 3 Results and Discussion
### Chemical Characterization
\begin{table}
\begin{tabular}{c c c c c} \hline  & **BC** & **EM** & **CON** & **Average** \\ \hline pH & 7.6 a ** & 8.1 c & 7.8 b & 7.8 \\ DM (\%) & 28.6 a & 30.4 a & 34.8 b & 31.2 \\ TP (\%) 1  & 0.43 a & 0.38 a & 0.33 a & 0.38 \\ TK (\%) 1  & 0.99 b & 1.41 b & 0.86 a & 1.08 \\ TC (\%) 1  & 22.0 b & 29.8 b & 16.5 a & 22.8 \\ TN (\%) 1  & 3.4 c & 2.6 b & 2.0 a & 2.7 \\ NO\({}_{3}\)-N (mg/kg) 2  & 628.8 c & 84.8 a & 414.2 b & 375.9 \\ NH\({}_{4}\)-N (mg/kg) 2  & 446.4 a & 380.9 a & 384.0 a & 403.8 \\ \hline \end{tabular}
\end{table} 
Table 4: Basic chemical characteristics of compots after seven months. AVE is the average value for a particular parameter. \begin{table}
\begin{tabular}{c c c c c} \hline  & **BC** & **EM** & **CON** & **Average** \\ \hline pH & 6.8 b ** & 6.1 a & - & 6.5 \\ DM (\%) & 28.5 b & 25.6 a & 29.4 b & 27.8 \\ TP (\%) 1  & 0.26 a & 0.28 a & 0.3 a & 0.28 \\ TK (\%) 1  & 1.7 b & 1.17 a & 2.11 c & 1.67 \\ TC (\%) 1  & 49.5 b & 50.8 b & 44.0 a & 48.1 \\ TN (\%) 1  & 2.8 b & 1.9 a & 3.0 b & 2.6 \\ NO\({}_{3}\)-N (mg/kg) 2  & 0.5 a & 1.0 a & 1.0 a & 0.8 \\ NH\({}_{4}\)-N (mg/kg) 2  & 170.3 b & 81.3 a & 257.3 b & 169.6 \\ \hline \end{tabular}
\end{table} 
Table 3: Basic chemical characteristics of input material. AVE is the average value of a particular parameter. Figure 4: Mean total carbon (TC) (**a**) and mean total nitrogen content (TN) (**b**) in DM of starting material and in mature compost with standard deviation in error bars for compost piles BC, EM and CON and average (AVE). Figure 3: Mean nutrient content in final composts. TN—total nitrogen, TP—total phosphorus, TK—total potassium with standard deviation in error bars. The average phosphorus content of fresh biomass after harvest was 0.28% in dry matter, with no differences between the piles. It increased by 35.7% to reach 0.38% of the dry mass of final composts. Comparing the TP content in 1 tonne of fresh material, average final compost contained (1.2 kg/t) less of it by half compared to cattle manure compost (2.2 kg/t). The highest phosphorus accumulation was observed in pile BC (from 0.26% to 0.43%), and the lowest in pile CON (from 0.3% to 0.33%). This characteristic, however, did not contribute to significant differences among final composts. The recommended P content in composts is 0.4-1.1% [43]; hence, pile CON fell short, while piles EM and BC met the requirement. Potassium content differed significantly among piles already at the beginning; pile CON had the highest potassium content, while pile EM had the lowest. Comparing the TK content in 1 tonne of fresh material, average final compost contained (3.3 kg/t) one third of the value in cattle manure compost (9.1 kg/t). While compost pile BC had by far the highest nitrogen content, compost pile EM had the highest potassium content (Figure 3). In pile EM, which was covered after one month, TK increased during composting, whereas it decreased in piles BC and CON. [PERSON] et al. [44] reported a similar trend in their study. It was found that TK decreased during composting with the exception of the closed system where TK increased after day 15 and then fell below the initial value in the substrate mixture. The expected values for compost potassium content are 0.6-1.7% [43], which was achieved by all three piles. The total carbon (TC) content of the starting material varied considerably; it was significantly lower in pile CON compared to the other two piles. In all three piles it decreased drastically from 48% to 23% of dry mass on average (Figure 4a) during composting, most of all in pile CON (by 62.5%). The findings are consistent with those of [PERSON] et al. [45], who claim that microbes can immobilise about 40% of available carbon because 60% is lost through respiration; therefore, a decrease in TC is expected. Compost pile EM had much higher TC than compost pile CON, whereas compost pile BC could be compared to both of them. The organic carbon content in all final composts, however, met the Slovenian regulation for first-class composts [46].