Source : Webster's Revised Unabridged Dictionary (1913)

Crumbly \Crum"bly\ (-bl?), a.
   EAsily crumbled; friable; brittle. ``The crumbly soil.''
   --Hawthorne.

Source : WordNet®

crumbly
     adj : apt to break into small fragments or disintegrate; "crumbly
           cookies"
     [also: {crumbliest}, {crumblier}]