If you know what you are doing, you can preserve papyrus very efficiently. The key is to keep it dry, hence all the survivals in desert locations. We have documents from Pharaonic times in our museums because of this (but practically none from Italy because of rain).
A sealed jar will only help so far, though. To be effective, it would need to be glazed pottery (uncommon in antiquity), stone, non-reactive metal or glass. A hypothetical Hellenistic Egyptian who would store scrolls in glass jars sealed shut with lead might well preserve his entire library (assuming the finders were reasonably good at recovering the material).
BTW, the most likely find spot for new and important papyri today is not the ground, but the storage vaults of universities, collectors and the antiquities market. People took them out of Egypt by the tonne in the late 19th and early 20th century, and many are still unread.