Mont. Code Ann. § 15-32-303

In addition to all other deductions from adjusted gross individual income allowed in computing taxable income under Title 15, chapter 30, or from gross corporate income allowed in computing net income under Title 15, chapter 31, part 1, a taxpayer may deduct expenditures for organic fertilizer and inorganic fertilizer produced as a byproduct produced in Montana and used in Montana if the expenditure was not otherwise deducted in computing taxable income.