Allowable Credit. A person operating an agricultural operation who sustains additional, but necessary, commercial fertilizer costs converting to a phosphorus based nutrient management plan, may claim a State income tax credit in an amount equal to 50 percent of these additional costs. Amount Allowed Per Taxable Year. For any taxable year, the maximum credit a person may claim is $4,500. If the credit exceeds the person's liability for the taxable year, the unused credit may be carried forward for up to 5 years. Limited Time Frame. A person may claim the credit allowed under this chapter for up to 3 consecutive years. Credit End Date. A person may not claim credit for any additional costs otherwise allowed under this chapter expended on or after January 1, 2009.