The problem with grass tracks is that the grass holds the dirt together . So when you are running on the grass its great but after one race the track is becoming really rutted because instead of just becoming loose the grass holds clumps of dirt together so you end up with clumps of earth in some places and loose dirt in others and its crap to drive on .
We had a grass track but as Brennan said we changed to astro turf with is just as grippy as grass, looks nice but has 0 maintenance and always stays consistant .
here it is
http://vimeo.com/12920958