Abstract: Effect of different combinations of nitrogen (N), phosphorus (P) and potassium (K) on six characters namely Root Length (RL), Root Fresh Weight (FRW), Root Volume (RV), Nodule Number (NN), Nodule Weight (NW) and Pod Fresh Weight (PFW) were studied in three lines such as Line-2, Line-16 and Line-18 of blackgram. Analysis of variance showed that all the three lines are different from each other for each of the characters. Factorial analysis showed that N and P after 15 days of sowing, N and K after 30 days of sowing, NPK after 60 days of sowing and P, NP and NK after 75 days of sowing had significant effect on root length. N and NK after 30 and 90 days of sowing and N after 45 and 75 days of sowing had significant effect on root fresh weight. For root volume, N after 45 and 75 days of sowing and N and NK after 90 days of sowing had significant effect. N after 15 days of sowing, PK after 30 days of sowing, P after 45 days of sowing, K and NP after 60 days of sowing, K after 75 days of sowing and NK after 90 days of sowing found to be significantly effective for nodule number. The character nodule weight also affected by K, NP and NK after 15 days of sowing, by P after 30 days of sowing, by N, P, K, PK and NPK after 60 days of sowing and by P, NP and PK after 90 days of sowing. Only P found to be significantly effective for pod fresh weight at an age of 75 days.