It was early recognized that one of the most important factors in soil analysis was the determination of humus. The analytical determination of the organic constituents present in soils and subsoils which should rightly be termed humus is a most difficult problem and one with which but little progress was made until recently.
The term humus as it is now quite generally used by chemists has a somewhat restricted meaning, referring only to that part of the organic matter which is extracted by treatment of a soil with dilute alkalies after the calcium and magnesium salts have been removed by dilute hydrochloric acid. Other meanings** have been given this term in soil literature.
Wells, A. A.; Stevenson, W. H.; and Coover, W. F.
"A centrifugal method for the determination of humus,"
Bulletin (Iowa Agricultural Experiment Station): Vol. 10
, Article 1.
Available at: http://lib.dr.iastate.edu/bulletin/vol10/iss124/1