Não informado. (1935). Soil conservation. U.S. Department of Agriculture, Soil Conservation Service.
Chicago-referens (17:e uppl.)Não informado. Soil Conservation. Washington: U.S. Department of Agriculture, Soil Conservation Service, 1935.
MLA-referens (8:e uppl.)Não informado. Soil Conservation. U.S. Department of Agriculture, Soil Conservation Service, 1935.
Varning: dessa hänvisningar är inte alltid fullständigt riktiga.