Rubbing dirt on hands scene in Gladiator

In her book Gladiator and Contemporary Society, University of New Mexico Professor of Classics Monica S. Cyrino writes:

When Marcus Aurelius asks Maximus after the battle in Germania: "Tell me about your home," Maximus delivers a sentimental speech about the simple beauty and tranquillity of his farm, with its fecund soil "black like my wife's hair." His wistful reverie evokes the modern individual's yearning for the simplicity of the land. Russell Crowe wrote this speech himself, drawing on his feelings of homesickness for his own ranch: "That's the way I feel about missing my home too."

Marcus Aurelius tells Maximus that his home is "worth fighting for" and thereby suggests that the protection of the small family farm is one of the purposes of Roman military conquest. The old emperor, beset by doubts about the legacy of his rule, has come to realize that the countryside, not the city, is the true Rome. Maximus' speech anticipates and fortifies his depiction as an old-fashioned man of the land in the rest of the film, one who has been brutally displaced. Maximus "has a farmer's vanity-free self-confidence." He picks up a handful of dirt and smells it before each fight, drawing strength from his connection to the soil. Franzoni explained why: "We wanted a character trait that humanized the hero before battle ... Some thought he did it when his life was in danger. But really, the impulse was, he does it when he's about to kick ass." Maximus cannot lose as long as he keeps in contact with the earth. (p.141)

David Franzoni was the screenwriter.

When I was with Dagara tribe in Burkina Faso, they taught me how they do shamanic work. They tap on the soil, to connect with the earth, and they speak with ancestors. Earth is a connection with ancestors.

And these very same elements were present in many indigenous cultures around the world. Before Christianity wiped that out.*

In Gladiator, Maximus probably had very similar culture. We might seen him praying to his ancestors, represented by wooden figures. His connecting with Earth by touching the soil is also a form of prayer and connecting with ancestors.

You can try it too. Just mere touching the soil outdoors has a strong grounding effect = it can get us out of chaos in our heads, connecting us with our very core and nature, Earth, ancestors.

May have been superstition.

Soldiers can be an incredibly superstitious lot. If Maximus had done that before his first battle and survived, me may have connected that in his mind with his survival. We see other examples of the character's superstition in the movie, which was certainly appropriate for the period.