The USDA is an agency created under the executive department by 7 USC §2201. The original department was created under Abraham Lincoln and many of the duties are now under the Food and Drug Administration. 

Westlaw has a four part complied legislative history of the Organic Food Products Act.

Since most agricultural regulation is via a federal agency the main ways that it is amended is though agency rulemaking and adjudication. However, agricultural law is also heavily influenced by the omnibus farm bill. While the farm bill is typically passed every five years, we are currently operating under an extension of the 2018 farm bill.