In order to be eligible for the Skyward Sword plow needed quest, you have to have finished the Fire Sanctuary and have completed all the jobs from Pumm (the owner of the Lumpy Pumpkin). To get the jobs, simply head to Lumpy Pumpkin and talk to him – and the jobs aren’t too difficult either!
To start the Skyward Sword plow needed quest, talk to Kina, the waitress at the Lumpy Pumpkin, during the day. She will be outside the Lumpy Pumpkin, at the pumpkin patch. She will task you in finding someone to plow her field.
This request will become a dowsing target. The one you need to help plow the field is Guld, the Mogma Elder. He is inside a volcano cone around the southern part of the Eldin Volcano (you enter it by parachuting up the steam sprout, and jumping down off the grate).
Talk to Guld and Fi will tell you he is a perfect candidate for the plowing job. Get Scrapper to take him to Kina, who will reward you with five gratitude crystals. Congratulations, you have now finished the Skyward Sword plow needed quest.