Mahdi Al-Kaisi, Mark Hanna, Michael Tidman (May 13, 2002). "Methods for measuring crop residue". Iowa State University. Diakses tanggal 2012-12-28.Pemeliharaan CS1: Banyak nama: authors list (link)
"Soil Compaction and Conservation Tillage". Conservation Tillage Series. PennState- College of Agricultural Sciences - Cooperative Extension. Diarsipkan dari versi asli tanggal 2017-08-03. Diakses tanggal 26 March 2011.
Ray Hilborn (date unknown). "Soils in Agriculture". University of Washington. Diarsipkan dari versi asli(PPT--available as non-PPT by searching the path through a search engine) tanggal 2015-01-12. Diakses tanggal 2013-08-28.Periksa nilai tanggal di: |date= (bantuan)
Ray Hilborn (date unknown). "Soils in Agriculture". University of Washington. Diarsipkan dari versi asli(PPT--available as non-PPT by searching the path through a search engine) tanggal 2015-01-12. Diakses tanggal 2013-08-28.Periksa nilai tanggal di: |date= (bantuan)
"Soil Compaction and Conservation Tillage". Conservation Tillage Series. PennState- College of Agricultural Sciences - Cooperative Extension. Diarsipkan dari versi asli tanggal 2017-08-03. Diakses tanggal 26 March 2011.