How do i know to use pressure canning instead of water bath canning?

You pressure can low-acid foods (pH > 4.6) to kill dangerous spores, and toxin in non-acidic environments. A regular water bath canner only reaches 212°F (100°C). A pressure canner allows you to achieve these higher temperatures by increasing pressure, making it essential for safely preserving low-acid foods like vegetables, meats, and poultry.

If you have a pH that is below 4.6 and must be processed in a pressure canner to reach the high temperatures needed to destroy harmful microorganisms. 

In a sealed container, adding pressure raises the boiling point of water above 212°F. A pressure canner increases this pressure to achieve the necessary 240°F temperature to kill the spores. 

Low-acid foods provide a favorable environment for Clostridium botulinum spores to germinate and produce a deadly toxin that causes botulism, a severe form of food poisoning. 

A pressure canner, when operated at the recommended pressure (usually 10-15 pounds per square inch), reaches temperatures of 240°F to 250°F. 

At higher altitudes, water boils at a lower temperature, so you must increase the pressure in the canner to reach the required 240°F.