Dunno about harvesting potatoes, but could he be confusing them with
turnips? Twain described losing his job as editor of a farming newspaper
because of his advice to a query on "When is the right time to pull my
turnips?"

Can't find it at the moment, and don't know if I have it here in Poland,
but I think it's a short called "How I Edited an Agricultural
Newspaper." Roughly, it's "if they're not ready for plucking from the
tree, let them ripen... Pluck them with a gentle twist - don't pull."

For the Clemens' 150th B-day at the Potrero Hill Public Library branch
in San Francisco, we called the all night reading "Turnips Should Never
Be Pulled" (and other stories). Never read any piece about plucking
potatoes, or we'd have use it.

Richard R (in Poland, where the lowly turnip (rzepa - pron "zhepa") is
very much a still-popular vegetable - very healthful, too - even tasty).