Or you can push a green thorn into a patch of dirt (not in a field), and it will produce a mix of reagents and fertile dirt. Sometimes, you'll only get regs (about 100 total), but other times you can get upwards of 50 fertile dirt from one thorn. A friend gave me a helpful hint: while waiting for the timer to reset before being able to push another thorn into the ground, grind the reagents into potions for your gardener to use. Sort of kills two birds with one stone.