Farmers Didn't Improve Their Fields Until the 20th Century?

One meme of a few in the food movement, Prof. Pollan I'm looking at you, is farmers began industrial agriculture in the 20th century, specifically when nitrates left over from the military started to be used on our fields.  (That's my memory of Omnivore's Dilemma.)

Low-Tech Magazine has a long post on lime kilns  (all that rain in the British Isles tended to acidify the fields, thus creating a demand for lime to counter it).   Wikipedia cites usage of lime for agricultural purposes in the 13th century.  It's easy to underestimate the brains of our ancestors.

I've memories of our whitewashing the stable walls, and using lime on the concrete behind the cows to keep it dry and prevent the cows from slipping.