Before every major battle (in Germania, in the arenas of Africa and in the Colosseum), Maximus bends down, grabs some dirt, and rubs it in his hand.
Now, usually we think of dirt as something you want to get off of your hands, so there's clearly something highly symbolic going down here.
First, this repeated action makes it clear that our friend Maximus isn't afraid to get his hands dirty. Literally and figuratively. This makes him, of course, the perfect general. He's not afraid to ride right into the thick of battle, or ride at the head of his cavalry.
The other important suggestions implied in Maximus' dirt-rubbing ritual is this: he's close to the natural world. He is, after all, a farmer who speaks lovingly of his rural home in Spain.
But his connection to the natural world also suggests that Maximus has heroic natural-ness. His predilections, loves, and desires are all portrayed as highly "natural." He loves his wife, adores his son, and fights with honor.
This natural-ness, of course, is contrasted against Commodus's unnatural acts. Not only is he a power-hungry psycho, but he also kills his father, and mandates that his sister give him an heir.