The digging bar, nicknamed the spud bar, is a very heavy steel bar with a wedged tip on the end. If you cannot find one, a long crow bar is a poor, but workable alternative. If you do not foresee rocks or roots as a problem, a crow bar may work. It would be a rare incident that no rocks or roots got in the way.
You need the heavy steel bar for breaking and prying out rocks and tree roots that may get in the way. The spud bar or crow bar is also used to loosen up hardened clay soil.