Salsa: the number one rated condiment in America!  And one of the most dangerous to can, if you don't take simple precautions.  I've seen time and again, someone without any canning experience say, "I've got this GREAT salsa recipe.  Friends love it!  I'm gonna can it!"  Typically, those recipes are for "fresh" salsas that you make by simply chopping veggies, add salt, drain and serve.  Those are NOT canned salsa recipes!

Acid is a primary ingredient for most canning recipes.  As a fruit, tomatoes are high-acid, but on the pH scale, they fall almost right on the border of low-acid vegetables.  And in a salsa, you mix them with more low-acid vegetables: onion, peppers, etc.  Which is why a safe canning recipe for salsa should include additional acid: vinegar, lemon juice, or lime juice.

To safely can salsa, it's best practice to use a recipe that has been tested by food safety experts.  Ball Blue Books have a nice assortment of recipes, as do many food safety departments of universities and state extension offices.  Never alter their recipes, other than spices, or substituting acids, e.g., switch lemon juice for vinegar.  And make sure to PROCESS each jar in either a water bath or pressure canner, according to directions.