Not sure about agricultural tax credit. But any reputable realtor should be able to tell you about the current use rules and regulations. It doesn't have to be agricultural land...just "open" land.
Unless this real estate bubble pops and totally collapses, you'd better have a barrel of money if you expect to buy land here.
I live in the DC area...so my trade here equals land plus house up there. The only thing I'm trying to avoid is the heavy property tax burden. Considering there are few other taxes, I shouldn't complain....but if I buy a property over say half a mil, I'll need a tax credit of some sort. I'll have to look on NH .gov site for info on the in current use credits. My idea was to plant some teeny xmas trees and declare myself a tree farm. I'd like to find about 25 acres with either an old house or a contemporary, possibly with some water frontage or at least a farm pond, anywhere in southern NH.