Everything You Need to Know about Writing in the Possessive
The simple rule: To form the possessive, add ‘s to words in every one of the first three categories:
2. Jones’s (except for Jesus, Moses, and other names from ancient history – nobody knows why; it just is)
Use only the ’ for words in category #4: boats’. The only exception is its which is already possessive; it’s is reserved for the contraction “it is.”
If you’re wondering: Farmers in the heading has no apostrophe because it isn’t possessive; it’s plural; like the turnips.