The lowest number of tubers per plant was recorded (5.3 tubers) in control. The highest number of tubers per plant was obtained from 600 kg ha-1 leonardite application (6.5 tubers), but no statistically significant differences were detected between 400 and 600 kg ha-1 leonardite applications (Table 3). Among the tested cultivars, Milva had the highest number of tubers per plant (7.7 tubers) and Agata had the lowest number of tubers per plant (4.4 tubers).