Specific gravity depends on dry matter content of tubers affected by starch accumulation and could be used as a quality trait (Hassanpanah et al., 2011). Even though fertilization is necessary for high yield, it prolongs necessary time for tuber maturity and excessive fertilization could cause tubers to accumulate excess water. Water accumulation reduces dry matter content of tubers and in turn specific gravity and quality of tubers (Herman et al., 1960). In the present study, specific gravity was higher in tubers harvested from leonardite applied plots than the tubers harvested from control. Similarly, it was reported that N containing organic fertilizers increase dry matter contents of potato (Jarvan and Edesi, 2009) and maize kernels (Ouggiotti et al., 2004)