How has the American agriculture changed during this last century?

What changed the quality of American agriculture (the greatest), in the 19th Century?

• Inventions, natural occurences, etc.

• Answer:

  Hemp, since it's been used to make the first american flag, also the american constitution, you can also make food from it, use it to build houses, for cosmetic and medical purpouses, etc. Hemp has over 50000 uses