Why is land not depreciated?

Not many would know the fact that Land differs from Field. there is no depreciation on Land but there is some kind of depreciation on Field or so i feel. Why is there no depreciation on Land when Land does decrease in value if we raise certain type of crops. i.e if we raise the crops of opium, most certainly the land will not be as suitable for raising rice as other lands. This will result in decrease in demand of that land and ultimately decrease in the price ( at least for a season).

tariqmomand b.com student from city university peshawar.