You could just split your solid, by pressing "shift+cntl+d" and start particle world that way. Or just keyframe the birth rate where you want it to start from 0 to whatever you need.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.