## 2 Materials and Methods  
### Experimental Design  
Stevia plants used in the experiment were planted on black degraded soil characterised by a humus content of 1.8%, salinity of 54 \(\upmu\)S/cm\({}^{3}\) and pH of 7.44 in the years 2014, 2015 and 2016; the values are given as the average of the years of the experiment. Stevia was planted at the Research and Didactic Station in Psary, Wroclaw University of Environmental and Life Sciences (51\({}^{\circ}\)19\({}^{\prime}\)055913609418\({}^{\prime\prime}\) N, 17\({}^{\circ}\)03\({}^{\prime}\)36781036313\({}^{\prime\prime}\) E). The experiment was performed using a one-factor randomised block design in triplicate on plots with an area of 1.5 m\({}^{2}\). Each of the field plots were separated from the other plots by an uncultivated plot of soil with a width of 50 cm. The total area of the experiment was equal to 33.75 m\({}^{2}\). The variable factor of the experiment was the spacing between the stevia plants, which was equal to 50 cm \(\times\) 30 cm (10 plants per plot), 45 cm \(\times\) 25 cm (12 plants per plot) and 30 cm \(\times\) 30 cm (15 plants per plot). The seedlings were obtained from a self-heated greenhouse, where seeds were grown in plastic boxes filled with peat substrate. Seeds were sown during the first ten days of April. Three weeks after sowing, stevia plants were transferred to 54-site seedling trays filled with peat substrate. Plants with at least three pairs of foliage leaves were planted between the 20 th and 30 th of May in 10 cm pots filled with peat substrate, and then, during the first ten days of June, plants were planted into the plots which were fertilized with ammonium nitrate at 100 kg N\(\cdot\)ha\({}^{-1}\) and covered with black agrotextile. The concentration of phosphorus in the soil was 45 mg\(\cdot\)dm\({}^{-3}\); the concentration of potassium was 310 mg\(\cdot\)dm\({}^{-3}\); the concentration of calcium was 2800 mg\(\cdot\)dm\({}^{-3}\); the concentration of magnesium was 34 mg\(\cdot\)dm\({}^{-3}\). An additional fertilizer in the form of ammonium nitrate (50 kg N\(\cdot\)ha\({}^{-1}\)) was used as a top-dressing four weeks after transplanting the seedlings into the field. Standard agronomic practices were followed for plant care, and irrigation was provided during dry periods at a rate of 20 mm of water per square meter of the plot (20 dm\({}^{3}\) per m\({}^{2}\)).  
Weather conditions at the site of the stevia plantation (temperature and amount of rainfall) during the experimental months are detailed in Table 1.  
Plants were harvested manually by cutting the stalk 10 cm above the ground during the second decade of September each year. After the harvest, the plants were weighed to assess the total yield of herbs and leaves (kg\(\cdot\)m\({}^{-2}\)). These data were then used to calculate the yield and the percentage of leaves in the yield. Leaves and shoots from individual plants were also weighed separately to determine their respective contributions to the total yield. Biometric measurements, including plant height, lateral width (measured at two-thirds of the plant height), total number of first-order shoots, and the total number of leaves on the lowest lateral shoot, were taken in July (during intensive growth) and just before harvest in September. The height of all plants was measured, and three healthy stevia plants of average height from each plot were selected for a complete analysis of biometric measurements (three per plot, nine per spacing, 27 per spacing over the 3-year experiment). Healthy, undamaged leaves (approximately 10-15 g per plant) were collected for the chemical analyses from the middle portion of the plant (at half the plant height). Measurements were taken three times per year in July and September for each field (nine per spacing, 27 per spacing over the 3-year experiment).