While this is a very old question, new information on Middle-Earthearth agriculture has just been published in The Nature of Middle-Earthearth, Part 3, "V: Note on Elvish Economy":
The Dwarves had an agriculture - which in early times they practiced when isolated and unable to buy grain etc. by barter. They had invented a "plough" of some sort - which they dragged as well as steered themselves: they were tough and strong - but they did not delight in such labor by necessity.