You have to set this in the surfaces.ini file in you track's data folder - Grass objects should normally be called "1GRASS_" in Blender, but you can also define your own surface types in the ini eg:
[SURFACE_5]
KEY=GRAVEL
FRICTION=1.4
DAMPING=0
WAV=
WAV_PITCH=0
FF_EFFECT=NULL
DIRT_ADDITIVE=0
IS_VALID_TRACK=1
BLACK_FLAG_TIME=0
SIN_HEIGHT=0
SIN_LENGTH=0
IS_PITLANE=0
VIBRATION_GAIN=0
VIBRATION_LENGTH=0
(These are just basic settings but play around to get the effect you want)
In this instance objects with the prefix "1GRAVEL...." would inherit these surface properties...