Artikel Jurnal
Agency for Agricultural Research and Development Ministry of Agriculture, 1992
Buku Referensi
Norton, George W., author
McGraw-Hill, 1993
Buku Teks
Mellor, John W., author
Cornell University Press, 1966
Buku Teks
edited by Melvin G. Blase
Iowa State University Press, 1971
Buku Teks