How do I depreciate a new irrigation well for farm producing grains?

You'll enter that in the Farm Assets section. The program will work you through the correct entry of the asset, and set up the depreciation schedule based on the selections you make and data you enter. Note that when asked for the cost, the cost includes the installation labor too, if you actually paid someone (other than yourself) to install it.

Water wells used in business have a recovery period of 15 years, use 150% DB.

Link is for IRS publication 225, see page 42;