Unless the taxpayer can prove that he/she has no retained economic interest in the minerals (dirt, in this case) and that he/she never depended solely on the sale of the minerals for a return of capital, the income would generally be subject to ordinary tax rates.

It would appear from the little information you provided that your client still has an economic interest in the dirt that has yet to be sold, which means income related to the sale of the dirt is likely ordinary in character instead of capital and could be royalty income.

You should probably look to whether your client is in the business of selling dirt as IRMN says, whether that's minerals extraction, and evaluate the nature of the expenses incurred to determine the proper tax treatment of those expenses (which may or may not require capitalization).