Leonardite applications increased tuber yield and yield increase associated with increased leonardite doses. Average tuber yield was 29.2 t ha-1 in control plots where only standard fertilization was practiced whereas tuber yields of 200, 400 and 600 kg ha-1 leonardite applications were 31.5, 33.2 and 33.5 t ha-1; respectively. While 400 and 600 kg ha-1 leonardite doses gave the highest total tuber yield, there was no statistical difference between these two doses (Table 3). The highest tuber yields were obtained from Milva (37.0 t ha-1) and Lady Olmpia (36.5 t ha-1) followed by Van Gogh (32.7 t ha-1) and Agata (21.3 t ha-1).