In the short story "A Deal In Wheat" by Frank Norris, naturalism is the basis of the theme but how would you describe the means in which he illustrates it?
Naturalism is a genre which highlights the power of nature over mankind. Under no circumstances will man ever be able to win out over nature, nature always has the upper hand.
This being said, the Naturalistic writer takes an observational role when writing the Naturalistic text. The author considers him/herself a scientific observer. They are simply recording events as they happen without interference.
As for Norris' short story "A Deal in Wheat", the most prominent illustration of how the text exemplifies the Naturalistic genre is the wheat itself.
The price of wheat is the overall most powerful "character" in the text. Given that wheat is a part of nature, the power that the wheat holds over all other aspects of the text is, by far, the most convincing example as to what makes the text Naturalistic. Regardless of what man/woman tries to do to change things, the wheat will always be the one in control.
As a side note, nature is typically personified in texts which depict Naturalistic characteristics. By doing so, the author is able to more directly assign power to the natural image depicted.