Source : Webster's Revised Unabridged Dictionary (1913)

Retentivity \Re`ten*tiv"i*ty\, n.
   The power of retaining; retentive force; as, the retentivity
   of a magnet.

Source : WordNet®

retentivity
     n : the power of retaining liquid; "moisture retentivity of
         soil" [syn: {retentiveness}, {retention}]