## 2 Materials and Methods  
### Study site, Experimental Design and Cultivation Conditions  
Field experiments using _Stevia reaudiana_ plants were conducted in the Wroclaw University of Environmental and Life Sciences: at the Research and Didactic Station in Psary (51\({}^{\circ}\)19\({}^{\circ}\)055913609418\({}^{\prime\prime}\) N, 17\({}^{\circ}\)03\({}^{\circ}\)36781036313\({}^{\prime\prime}\) E; 113 m over the sea level). Stevia plants used in the experiment were planted in black degraded soil characterised with humus content of 1.8% in the years 2014, 2015 and 2016. Table 1 shows the temperature and amount of rainfall during the time of the experiment at the stevia planting area.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Month** & **Decade** & \multicolumn{2}{c}{**2014**} & \multicolumn{2}{c}{**2015**} & \multicolumn{2}{c}{**2016**} \\ \hline  & & [\({}^{\circ}\)**C]** & **[mm]** & [\({}^{\circ}\)**C]** & **[mm]** & [\({}^{\circ}\)**C]** & **[mm]** \\ \hline \multirow{4}{*}{May} & 1 st & 11.4 & 30.3 & 13.3 & 12.3 & 15.4 & 14.0 \\  & 2 nd & 12.2 & 33.6 & 13.0 & 14.0 & 15.5 & 12.0 \\  & 3 rd & 17.3 & 42.8 & 14.0 & 0.5 & 20.9 & 0.0 \\  & Monthly average & 13.8 & 106.7 & 13.4 & 26.8 & 17.3 & 26.0 \\ \hline \multirow{4}{*}{June} & 1 st & 18.4 & 8.2 & 18.1 & 11.0 & 21.1 & 9.4 \\  & 2 nd & 16.6 & 0.3 & 16.4 & 23.1 & 18.5 & 59.0 \\  & 3 rd & 15.7 & 15.4 & 15.8 & 32.2 & 23.8 & 0.8 \\  & Monthly average & 16.9 & 23.9 & 16.8 & 66.3 & 21.1 & 69.2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Rainfall and temperature on site of the experiment during the time of the stevia growth.  
The experiment was performed in a one-factor randomised block design in quadruple repetitions, on the plots with the area of 1 m\({}^{2}\) with the total of sixteen plots. Total area of the experiment (experimental plots and uncultivated areas of the field used to divide the experimental plots) was equal to 33.25 m\({}^{2}\). The variable factor of the experiment was different types of irrigation practice. Four plots were not irrigated at all (0 W), four plots were irrigated once a week (1 W), four plots were irrigated two times a week (2 W) and four plots were irrigated only during the dry periods (no rainfall: NR) in the period from three weeks after planting the seedlings into the soil up to the harvest (phase of the intensive growth of stevia plants). The amount of water used during each watering (for the samples NR, 1 W and 2 W) was equal to 20 mm. The seedlings were obtained from our institution's own heated greenhouse, where seedlings were grown in plastic boxes filled with a peat substrate. Seeds of _Stevia reboundana_ 'Sweet Herb' variety were sown in the first decade of April. Three weeks after sowing, stevia plants were transferred to 54-cell plastic trays filled with a peat substrate. In the second decade of May, plants with at least three pairs of foliage leaves were planted in 10 cm pots filled with a peat substrate. Stevia plants were planted during the first decade of June into the plots, which have been preplant-fertilised with ammonium nitrate at 100 kg N-ha\({}^{-1}\) and a multiched black agrotexile. In the third decade of July, plants were top-dressed-fertilised with ammonium nitrate (50 kg N-ha\({}^{-1}\)). The concentration of phosphorus in the soil was equal to 45 mg-dm\({}^{-3}\); concentration of potassium in the soil was equal to 310 mg-dm\({}^{-3}\); concentration of calcium was equal to 2800 mg-dm\({}^{-3}\); and concentration of magnesium was equal to 34 mg-dm\({}^{-3}\).