This article is largely based upon the work conducted during the last six years by the writer while he was connected with the Michigan and Iowa Agricultural Experiment Stations.] UP to a few years ago the generally-accepted idea was that humus is made up of but a few organic compounds, chiefly acid in their nature. However, the more recent investigations have thrown enough light upon the chemical nature of humus or humus organic matter in the soil to demonstrate that it is a very complex substance which, in addition to the dark-colored humin bodies, contains a large number of organic compounds displaying acid, basic, neutral, and amphoteric character.
Considered from a modern point of view, the solution of the humus problem consists, first of all, in finding out all of the compounds--the simple bricks--of which humus is made up, as well as the proportion in which those compounds are mostly present in humus, and, further, to ascertain just which of the compounds in question are, as such, present in humus and which of them constitute parts of one or several more complex bodies.
What we at present designate as soil organic matter or humus will thus, in the course of time, by modern methods of research, be separated into a number of bodies of well-known composition and structure.
Whereas a part of the above problems may now be considered as accomplished--the results in question will be mentioned later-it may be worth while to show here that in some measure the development of the idea of the chemical nature of humus stands in a certain relation to the development of the chemistry of carbohydrates and proteins out of which humus is being formed in the soil.
For centuries mankind knew only cane-sugar or saccharose, which was originally extracted exclusively from the sugar-cane. The same sugar was later discovered in the sugar beet (Marg-* Communicated by the author and published by permission of the Secretary of Agriculture.
VoL CLXXVI, No. lO55--4o 565 566 S.L. JODIDI.
[J. F. I.
graf, 1747) , sorghum, maple tree, and other plants. In addition to this sugar, there were discovered, also, lactose in the milk (Bartoletti, I619) , glucose (Lowitz, I792 ), fructose (Dubrunfaut, I847), etc., so that at present we know a considerable number of sugars in the form of bioses, trioses, tetroses, etc., up to nonoses,--i.e., sugars which contain in their molecule from two to nine carbon atoms respectively, and which in part occur as such in nature, the pentoses and hexoses and the corresponding polysaccharides being the most important ones.
As was demonstrated by many researchers, such sugars and, generally speaking, carbohydrates, 1 when treated with acids or alkalies, yield brown or black humin substances, whose physical and chemical properties remind one of soil humus to such a degree that the artificial and natural products were considered by some as closely related, 2 by others even as identical.
When, however, dilute acids are applied, the carbohydrates yield a number of well-defined intermediary products. Thus the polysaccharides furnish, first, monosaccharides ; these latter yield organic acids, etc. For the raffinose, 8 e.g., we have: Raffinose (melitriose)
~ melibiose (+ d-fructose) ~ d-glucose + galaetose. The monosaccharides obtained can yield, e.g., lactic 4 acid, butyric ~ acid, alcohol, 6 citric acid, etc., depending upon the conditions of fermentation.