When you knock your foes into the sand, they have a hard time regaining their feet.
Prerequisites: Combat Expertise, Improved Trip.
Benefit: If you successfully trip an opponent in any area of ash, dust, loose earth, or sand that is at least 1 inch deep, that opponent must take a full round action to stand during the round after being tripped. Following that round, the target can get to his feet with a move action (as normal). The opponent still provokes attacks of opportunity while attempting to stand, as normal.
See Determining Loose Soil for finding the chance of sufficient ash, dust, grit, loose earth, or sand existing to use this feat.
Source: Sandstorm, page 52.