There are numerous sources for the history of American agriculture, including the United States Department of Agriculture (USDA), the Smithsonian's American History Museum, Google Scholar, or a specific university's history department. For example, Texas A&M (Agricultural and Mechanical) has much historical research available.
The Morrill Act of 1862 provided for donations of public lands to fund colleges whose "leading object" was teaching "branches of learning as are related to agriculture and mechanic arts." Many colleges were founded with those donations for that purpose, and they are a rich source of historic materials.
Many universities also offer classes in agricultural history that go into great detail about government farm programs, statistics on acreage dedicated to specific crops and historical output, trade balances and protective tariffs. There is even research on the uses of different farm equipment such as tractors, combines and reapers.