## 2 Materials and Methods
### Experimental Site and Design  
The study was carried out in Rieti, Italy (latitude 42 deg24'29\"52 N, longitude 12 deg51'36\"36 E) during the period March-August 2017. The sampling site is characterized by relatively cold and rainy winters and hot and dry summers. The long-term average annual temperature recorded between 1980 and 2009 was 12.1 degC; the mean temperature during the growing season was 16.0 degC; the average maximum temperature for the summer months was 30.1 degC; the average minimum temperature for the coldest months was -1.6 degC. In summer, maximum temperatures were often over 30 degC. The annual mean precipitation was equal to 1021 mm and it fell mainly from October to December. Summer rainfall was irregularly distributed and the total mean amount over June-August was 136 mm. In 2017, the annual average temperature was 12.7 degC; the average temperature in the growing season was 17.4 degC; the average maximum temperature for the hottest months (July to August) was 33.2 degC. The annual precipitation, precipitation for the growing season, and precipitation over the hottest months was 721 mm, 186 mm, and 27 mm, respectively. The annual relative humidity was 68%.  
A completely randomized experimental design was conducted during the hop growing season in a climate uncontrolled greenhouse environment from March to 5 May and outside from 6 May to 28 August. Three different cultivars of German hop plant (Hallertau Magnum, Perle, Spalter spalt) were considered for the study due to their economic importance and their sensitivity to environmental conditions [27]. These three varieties grow well in most climates, but best in warm, dry and sunny regions. Hallertau Magnum produces very good yields (1700-2300 kg ha\({}^{-1}\)); the cone's structure is very large and longish size. Perle also produces good yields (1600-2100 kg ha\({}^{-1}\)); cone size is small to medium. Spalter spalt yield amounts 1750-2000 kg ha\({}^{-1}\); the cone size is small to medium.  
In March, hop rhizomes were placed in plastic pots (15 cm \(\times\) 14 cm) with soil containing acid peat, expanded perlite and clay. Each pot contained 8 L of soil. The packed pot soil had a pH of 6.0, a dry bulk density of 1.1 g cm\({}^{-3}\), an electric conductivity of 0.45 dS m\({}^{-1}\) and a porosity of 90% \(\
icefrac{{\eta}}{{\upsilon}}\). Hop rhizomes for each variety were randomly assigned between pots with soil added with 20% of biochar (1.6 L of biochar and 6.4 L of soil containing acid peat, expanded perlite and clay) and pots without biochar (control). Trials with biochar were replicated seven times, whereas controls (without biochar) were run in triplicate. We chose an unbalanced sampling to better evaluate the variability of the biochar effect on hop plants. Young plants were transferred from pots to the field at the beginning of May, corresponding to the moment of bines elongation. No biochar was added in field. Until June, the plants were irrigated with 0.5 L of water every two days; in the two warmest months (July and August), the same amount of water was given every day.  
Plant development was monitored through the percentage of roots in pot (as described in the Section 2.4) and the number of leaves at the beginning of May; shoot diameter was measured by caliperat five different times (T1: 30 June, T2: 18 July, T3: 27 July, T4: 3 November, T5: 21 November), i.e., before flowering, near cones maturity, before plant dormancy; measurement of climbing bines length and number of cones at maturity.