George Washington Carver comes to mind.
Eli Whitney, perhaps.
Whoever came up with crop rotation.
Whoever came up with growing hops, malt and barley...
Seriously, though, I would put Carver at the top of the list.
I can add to this list daily:
First to come to mind is Archimedes, born in 287 BC. he invented "Archimedes Screw" the first helical screw used to raise water for irrigation.
The very first is probably Cro Magnum, early man used a stick to scratch the ground for planting and digging.
Also, nineteenth century, Cyrus McCormick (and Obed Hussey)for the reaper (1834 and 1833 patents)and John Deere for the steel plow (1837). McCormick's Son founded International Harvester.
The First true agricultural machine (opinion) is the seed mill by Jethro Tull in the late 1600s, also he invented the Horse Hoe.
The first powered tractor, 1892 John Froelich.
The Mormons in Utah, 1847 standardized modern irrigation practice (techniques).
Scottlands Robert Bakewell developed the Leicester sheep, the drainage of land using pipe tiles.
Carl Linde developed the ammonia compressor that made it possible to freeze food food for long shipments (1876).
Nicolas Appert for canning and Louis Pasteur for pasteurization.
