Cooking food and dung in the Cauldron will give you fertilizer. When used on Farmland, it will turn it into Fertile Farmland. This will speed up the growing process. After a while, this land will turn back into normal Farmland (this is random). You need to use Fertilizer (or Bone Meal) to turn the Farmland back into Fertile Farmland.
Fertilizer is easier to get than bone meal in the first few days/weeks of your world.