A plowed tile is ready for planting. When you harvest a crop from a tile, it becomes smooth. Press
to plow the tile, which will make the tile have lines.
The previous solutions of plowing and recycling a tile will work fine. I did want to point out that for most of the achievements in this game, you don't have to be on your own farm to make progress. To prevent as much of the unholy grind for this, visit other farms and see if there's anything to plow there. Along with building your boost meter to give you double resources and XP when you go back to your own farm and harvest things, it is significantly less boring.