References

·        Kathy Einwich(2004): http://www.epa.gov/gmpo/edresources/soil.html

·        Stephen W.Broome: http://www.soil.ncsu.edu/lockers/Broome_S/ssc051/index.html

·        Bell R.W: http://www.ldd.go.th/Wcss2002/Abstracts/1614.pdf

·        L. S. Sonon, M. A. Chappell: http://www.agron.iastate.edu/soilchemistry/History%20of%20Soil%20Chemistry.htm#In%20the%20Beginning

·        Fred Moor(1999): http://soil.hostweb.org.uk/

·        Eric Rymer(2000): http://www.historylink101.com/lessons/farm-city/story-of-farming.htm

·        U.S. Dept. of Agriculture: http://www.epa.gov/gmpo/edresources/soil.html