An experiment was carried out to study the effects of Time Factor, potassium and cobalt on Rhizobium growth. The objective of the experiment, which conducted under laboratory conditions, was to investigate the interaction effects of using three levels of cobalt (0, 0.375, 0.75 mg Co. Kg^-1 sterile soil) and four levels of potassium (0, 25, 50, 100 mg K . Kg^-1 sterile soil) on the viable counts of Rhizobium growth in the Silt Clay loam soil after 3, 6, 12 and 18 days of incubation at 28°C. The results indicated that cobalt level 0.375 mg Co. Kg^-1 and potassium level 50 mg K . Kg^-1 recorded the highest significant increase in the viable counts of Rhizobium growth in the sterile soil especially after 12 days of incubation at 28^°C which got 24.00 × 108 cell.gram^-1 dry soil.