I loved doing this when I was learning how to read and write! Have the child say the individual letters as they trace them in the salt/cornmeal, and you're addressing all three learning styles: visual, auditory, and kinesthetic.
Exploring Kinesthetic/Tactile Learning Styles - Some people find they learn best through reading, some through hearing, and others through doing. Learning style can affect how well you come to understand classroom material, your interest in and enjoyment of a subject, and even what you choose as your later career. For more information about hands-on programs for kinesthetic/tactile learners, visit: http://www.pti.edu/programs