Andrew,
The grass will burn off when the lead melts, the dirt will float to the top as will all the other lighterwieght impurities. Spoon that stuff off, flux the lead to recombine the tin and antimony into the lead, and cast away. The tin makes the bullets fill out the mold more consistently; the antimony makes it harder.
Cowboy shooting shouldn't have any problem with your material and unless you are shooting for the ultimate in accuracy, as in bullseye, you are on your way to some good and inexpensive shooting. Scrounging is an effective way to gather lead.
Fash