Farming in the United States reached a turning point about 50 years ago. Up to that time, increasing amounts of labor and land had been used to produce most of the incease in farm output.
Tweeten, Luther G. and Heady, Earl O.
"Agriculture in 1980 - A Look Ahead,"
Iowa Farm Science: Vol. 18
, Article 4.
Available at: https://lib.dr.iastate.edu/farmscience/vol18/iss10/4