Farms have decreased because an individual can make a better living elsewhere, with less effort, and methods have changed.

Improper use of land has destroyed uncounted millions of acres, now salinated, deserty, eroded, mesquite and weed-covered, and useless to man or beast. Concentration of holdings offers some hope that the economic means may be found for attempts at restoration. Unneeded land can be returned to the government.

The number of farms has decreased 63% since 1900 and farm size has risen 67%.

1900- 41% of workforce employed in agriculture
1930- 21.5% of workforce employed in agriculture, AG share of GDP 7.7%
1945- 16% of labor force employed in agriculture
1970- 4% "    "    "      "
2002- 2%   "   "    ". Ag share of GDP 0.07%

Economic Research Service, USDA
http://www.ers.usda.gov/publications/eib3/eib3.htm#changes