Pretty sure both the baking soda and plaster of paris tricks are both Internet myths. It has been discussed on here before, and can't recall anyone having any luck with either one.
What does work is remove all sources of food they have access to, which is why you have rats in the first place. If you can do that....starve them out.....they will be forced to leave or die. This is the first step in getting rid of your rats, which is sanitation. BYC member Al sells his rat proof chicken feeder, which I use, and which does a good job of limiting feed to the birds and none to the rats. No spillage either. That alone may do it, but you also have to keep them out of bulk feed in storage, and the hard core rats may try to eat eggs, or even droppings to survive.
But if they are that hungry.......they will then be receptive to the poison bait blocks, which are ALWAYS served up from secure (meaning locking) bait stations, so only rats and mice can get at the blocks, and bait blocks are also pinned in place in the bait stations, so rats can't pull them out to carry them off. This is the elimination part, which always comes last.
And in case you are wondering, trapping doesn't work either.....will barely put a dent in them. Can be used to monitor the situation (but so can a game camera).