THE OBGANIC MATTER OF THE SOIL 131 



been mentioned. It is to be kept in mind that the simpler 

 products of decay are the ones utilized by crops, although 

 it is a well-established fact that some of the secondary 

 and intermediate compounds may be taken up by certain 

 plants and probably are of some importance from the 

 standpoint of use as plant-food. 



91. Composition of the soil humus. — It is evident 

 that the most complicated parts of the organic matter 

 in the soil are the primary and the secondary products 

 of decay, or the so-called soil humus. The stud}'' of this 

 matter is difficult and calls for the very highest knowledge 

 of organic chemistry. This is true for two reasons: 

 first, because of the complexitv of these compounds; 

 and, .eeondly, because they ar; continually changing. 

 A certain compound present in the soil one week may 

 disappear the next week. Moreover, while some of the 

 soil humus is soluble in water and may circulate in the 

 soil solution, the bulk of it is insoluble. This in itself 

 presents difficulties. When the soil humus is treated 

 with the various extractive agents, reactions may be 

 induced which would not take place in a normal soil 

 Compounds are then formed which not only would be 

 abnormal, but would probably not exist mider natural 

 conditions. 



A great many chemists have worked on the problem of 

 the constitution of the organic matter of the soil and 

 have published their results. The ideas of the early 

 workers are really embodied m the conclusions advanced 

 by Mulder,^ who was in many ways far in advance of his 



1 Mulder, T. J. Die Orgamsehen BestandtMle im Boden. 

 Chemie der Aekerkrume, I, pp. 308-360. Berlin, 1863. Also, 

 Wiley, H. W. Agricultural Analysis, Vol. I, p. 53. Easton, 

 Pa, 1906. 



