"What changes in agriculture occurred between 1700 and 1900?"
This happened.

http://en.wikipedia.org/wiki/Plough

The basic plough with coulter, ploughshare and mouldboard remained in use for a millennium. Major changes in design did not become common until the Age of Enlightenment, when there was rapid progress in design. Chinese ploughs, with mouldboard, were brought to Holland in the seventeenth century by Dutch sailors. And because Dutchmen were hired by the English to drain the East Anglian fens and Somerset moors at that time, they brought with them their Chinese ploughs. The English called these Chinese ploughs the 'bastard Dutch ploughs' instead of 'Chinese ploughs'. Thus, the Dutch and the English were the first to enjoy the efficient Chinese ploughs for the first time in Europe. The Chinese-style ploughs were spread to Scotland from England, and from Holland to America and France.[12][7]

"What changes in agriculture occurred between 1700 and 1900?"

Don't forget my prior posts on the UK deadheading 3.5 million 'immigrants'/year as O-NPK deadweight tons, mummified Egytian cats by the pitchfork ton, and the lethal grab for naturally superphosphated seabird & bat guano, plus Atacama Desert Nitrates sailed across the blue seas.

The UK had quite a bit of imported O-NPK topsoil enrichment before the industrial development of Haber-Bosch ammonia & urea [natgas-sourced N-products], and the logical mining enlargement of the P & K mines from pick, shovel, and wheelbarrow infrastructure to the subsequent giant draglines, multi-mile conveyor belts, multi-head rock-grinders, huge 3500 ft deep multi-ton lift shafts, and giant beneficiation factories using globally moved megatons of sulfur to make phosphate ores in water-soluble finished products.

Recall prior postings whereby a 40 lb bag of high potency [High NPK ratio] I-NPK may have 5 gals of gasoline equivalent energy embedded into the product when it is finally dispersed to the final topsoil square foot.

Have you hugged your bag of NPK today? There are No Substitutes to the Elements NPKS to leverage photosynthesis above a Liebig Minimum!

Bob Shaw in Phx,Az Are Humans Smarter than Yeast?