Columbia subject headings:
Agricultural innovations--United States--History--19th century.
Agriculture--United States--History--19th century.
Harvard subject headings:
Agricultural innovations--United States--History--19th century.
Agriculture--Economic aspects--United States--History--19th century.
Agriculture--United States--History--19th century.
Princeton subject headings:
Agricultural innovations--United States--History--19th century.
Agriculture--United States--History--19th century.