As you have clearly stated, you must look to the primary purpose of the loan to determine if it is covered. If the purpose of the loan is both consumer and agricultural with neither primary over the other, I would take the conservative approach and treat it as a consumer loan in order to avoid any issues later on. (See the commentary below.)
1026.3(a) Business, Commercial, Agricultural, or Organizational Credit
1. Primary purposes. A creditor must determine in each case if the transaction is primarily for an exempt purpose. If some question exists as to the primary purpose for a credit extension, the creditor is, of course, free to make the disclosures, and the fact that disclosures are made under such circumstances is not controlling on the question of whether the transaction was exempt.