To keep a journal is to write in a journal regularly. In this case, it means he wrote in his journal consistently for more than fifty years!

It means to write a journal/diary. In this context he kept/wtote his journal to keep track/record his observations/conclusions in order to use them later for something, maybe for the purpose of research.

A journal is like a diary. Some people write their thoughts and others might make it into a scrapbook or creative project. "He kept a journal" is like saying "he wrote in his diary" I think the 'kept' part comes from the fact that he didn't give up. He kept writing in his journal. He kept his journal. In this example he has written in his diary for 50 years.

'To keep a journal' means to write in a diary regularly.

You are welcome.:)