Go to your township assessor and talk to him about it. His name and phone number is on your annual assessment notice. Ordinarily, its going to be based on your zoning since they are required to assess you at the "highest and best" use for your property. Also, while Ag property is usually assessed lower, I donʻt remember an Ag tax exemption for real estate taxes. Only sales tax is exempt for farm use in Michigan and then all you have to do is sign an "exempt purchase" form that the seller will supply you at the time of purchase. But I moved away from Bangor/So. Haven about ten years ago, so I might be wrong.
I work for the County Assessor/Treasurer (ours does both), and if I'm not mistaken, I think it's the Treasurer side that does the tax exemptions. (I could be wrong about that). The exemption is a great thing, but make sure that you don't have any plans to sell your property in the near or distant future, as there are some pretty stiff penalties if the land gets taken out of tax exempt status. In my county if you take the property out of the program you have to pay 7 years of the full amount of back taxes on the full market value of the property. It's a great program to keep land open and undeveloped.