You can plow, seed, and harvest one plot at a time by default; however, if you purchase farm equipment you can then do each 4 at a time. The only draw back is they require fuel. Your allotted only a maximum amount of fuel. Currently that amount is 150. This means you can plow 150 squares or seed 150 squares or harvest 150 squares. Your fuel is then replenished at the rate 1 per 2 minutes.

Each piece of equipment costs 30,000 coins.

You can buy the Tractor(plows) at level 12 it comes in 2 colors red and pink.

You can buy the Harvester at level 13.

You can buy the Seeder at level 14.

Recently to help consolidate their properties zynga as added crossover items. 3 of these are farm equipment. The tractor, Havester, and Seeder can do 9 square with each click(3X3 area). To get access to these you have to reach a certain level in mafia wars