Wash and drain asparagus spears. Leave spears whole or cut into pieces. Boil 3 minutes. While hot, place into canning jars, leaving 1 inch head space. Add 1/2 teaspoon salt to pints, 1 teaspoon salt to quarts, if desired. Cover with boiling water. Adjust caps. Process in steam pressure canner 25 minutes for pints, 30 minutes for quarts, at 10 pounds of pressure (240° F). Keep canned asparagus on a shelf, in as cool and dry a place as possible. Date jars as you prepare them and use the oldest jars first. Use within a year.