He does this to privately mourn for Luis.
Paul's seemingly odd actions are the subject of the chapter titled "Thursday, November 30." The blue suit was the one he wore to Mike's funeral, so for Paul, he associates those clothes with mourning and respect. The timing is important because Paul needs privacy to do this: his mother had left the house that morning, so Paul is alone.
He goes outside without any idea of how he will honor Luis, just knowing that somehow, he must. Paul upturns a rectangle of sod and sees how the earth is sandy beneath it. For Paul, Luis, the knowledgeable and kind young man who had tended the citrus groves, will always be associated with the earth and with trees. Paul contemplates this connection and cries for Luis's death, letting his tears flow straight into the sandy ground. At the same time, Paul ponders how thin the veneer of the sod is over the real ground, inviting the reader to consider how thin the lies and explanations are over the real truth of the story--like who's responsible for Luis's death, and for Paul's eye injury.
After this cathartic act, Paul feels odd but "remarkable"; he feels that Luis is now a part of him.