What crop has been farmed in large amount on the land?

If your question pertains to Chapter 11, the only crop mentioned is cotton.

For nitrates are not the land, nor phosphates; and the length of fiber in the cotton is not the land.

In Chapter One, both cotton and corn are mentioned.

"Ever' year," said Joad. "Ever' year I can remember, we had a good crop comin' an' it never come. Grampa says she was good the first five plowin's, while the wild grass was still in her." The road dropped down a little hill and climbed up another rolling hill.