How much explosive force would it take to create a mushroom cloud?

The answer depends on the yield of the explosive used in the device.

The ryder rent-a-bomb that the Oklahoma city bomber used caused a mushroom cloud and that was just fuel-fertilizer mix.

If you wanted to replicate a bomb the size of  a 1 Kiloton nuclear warhead yield it would probably require a connex container’s worth or about a semi-truck-trailer. That is if the explosive was a low yield fuel-fertilizer mix. Of course the nuclear warhead would be incredibly smaller.

 

There are many things that can go boom, including flour silos and grain elevators.