# Greenhouse Residues' Potential for Biogas Production
## 3 Results  
### Physicochemical Characteristics  
The characteristics per residue type (tomato leaves and suckers and cucumber leaves and suckers) along the three sampling times are shown in Figure 1. The TS content of the leaves was not statistically different over time, except for the cucumber leaves in March, which seemed different from the samples taken in May (\(p\) = 0.012) and August (\(p\) = 0.042). The suckers seemed to be drier in May; however, the TS of the samples in May did not differ statistically from the samples in August (\(p\) = 0.052) but differed from the samples in March (\(p\) = 0.028) in the case of tomato sucker. In the case of the cucumber suckers, there was a statistical difference between the samples of May and August (\(p\) = 0.004) and between May and March (\(p\) = 0.010). The same trend in the variation was observed in the VS content of all types of biomass. The TKN content was statistically the same in time in most cases (\(p\) = 0.063 to 0.916), except for the samples taken in March and May for the tomato suckers (\(p\) = 0.013) and in March and August for the cucumber suckers (\(p\) = 0.010). On the contrary, the COD content was statistically different in most cases; tomato leaves and suckers increased their COD content from March to August, while the cucumber leaves and suckers had lower COD in May and statistically similar COD in March and August (\(p\) = 0.082 and \(p\) = 0.096 for leaves and suckers, respectively). It should be noted that the tomatoes were planted in February and remained until the end of the season (October), while the cucumbers were planted twice (in January and June). Therefore, the plants were younger in March or August than in May. The phosphorus content of the samples in time was statistically the same only in some cases, that is, between March and August (\(p\) = 0.255) for the tomato leaves, between March and May (\(p\) = 0.871) for the cucumber leaves, and between March and August (\(p\) = 0.813) for the cucumber suckers. In all other cases, there was a significant statistical difference within the same type of plant residue, but no safe conclusion can be drawn regarding how the plant maturity phase or temperature correlate with the P content of the leaves and suckers for each crop. For example, the leaves of tomato seemed to be richer in P during May, while the leaves of cucumber had the highest P content in March and May. Moreover, suckers of tomatoes had the highest P concentration in March, while the suckers of cucumbers contained the highest P in March and August. On the other hand, the cultivation practice applied with respect to fertilization, which could affect the P content, did not change during these months. More frequent sampling events within March, May, and August could reveal the variance of P content in these months accurately. Averaging the values of each parameter within the same residue type resulted in a high standard deviation in the cases for which significant differences in time were noted (Table 2). For example, the TS and VS content of suckers, which were much higher in the samples taken in May, had a standard deviation of 20-30% of the mean value. The same applied to the cucumber leaves with a higher TS content in the samples of August. As a result of the high standard deviation, there were no statistically significant differences in the TS content of the various residues (except for tomato leaves being drier than the cucumber leaves, \(p\) = 0.007). The VS content followed the same trend. The COD content had a low standard deviation (except for the tomato suckers being the richer in COD in the samples of August). As a result, the comparison of the means revealed that the tomato leaves had a lower COD content. However, it was not statistically different from the COD content of the tomato suckers, which had a higher standard deviation (\(p\) = 0.178). Similarly, the TKN content had a low standard deviation (4-10% of the mean value) and most values were statistically different (\(p\) < 0.05).