Let me preface this by saying I have very little experience with loot tables and I'm not really sure how they work. I have, however, made a few data packs, so as far as I know I'm doing it correctly.

I wanted to make a data pack that changed composters to give dirt instead of bone meal. I extracted the vanilla loot table for composters, and I changed the line that gives bone meal to give dirt instead. No other changes. However, it still produces bone meal.

Just to be sure, I changed the "explosion_decay" drop to a diamond block. I then blew up a composter with TNT, and sure enough, it dropped a diamond block. That makes me positive I've at least made the data pack correctly.