There otherwise means "if potash had not been used so much in manufacturing because it served so many industrial uses and as a result was exported to market". The word refers to a condition or situation mentioned in the context and means "if that condition or situation did not exist or if it was not true".
She must have had no coins with her when she parked her car. Otherwise
she would not have looked at the time on the meter, gone into the shop, and then come out of the shop 30 seconds later and put a coin in the parking meter, only to go right back into the shop again.
There, otherwise = if she did indeed have coins with her when she parked her car.
otherwise is a synonym for else.
If the sky is gray and cloudy bring your umbrella, otherwise leave your umbrella at home.