Plum Sauce

[Southern Scarab]

The plum farm was struggling
since it’s plumbing had burst.
Old man johnson was sour
but was not well rehearsed,
like a plum that’s still green
or a poet's first verse;
he resolved to expunge
all the plums from the earth.

I love the consistent double meanings It's very balanced and well written, but why isn't Johnson capitalized?
Thanks for the read!