TRANSFORMATION OF ORGANIC MATTER 673 



as sources of energy by various microorganisms. Even different alde- 

 hydes and other compounds toxic to plant growth are decomposed in 

 the soil, under favorable conditions. 19 



Some of these numerous compounds may be present in the soil in 

 very low concentrations or as mere traces, or may be absent altogether. 

 Most of them are probably formed in the processes of extraction of 

 these materials from the soil and are not present in normal soil. The 

 very nature of the larger part of the soil organic matter, which is com- 

 monly referred to as "humus" and which is only very slowly decom- 

 posed in the soil is still little understood, notwithstanding the extensive 

 literature dealing with this subject, as shown later. To be able to 

 understand the nature of the soil organic matter, it is essential to know 

 the processes of decomposition of the various constituents of the natural 

 organic materials added to the soil. Without a knowledge of these 

 transformations, the phenomena leading to a change from the organic 

 substances of plant or animal origin to a group of complex bodies in the 

 soil, very resistant to decomposition, can never be explained. 



Decomposition of organic matter added to the soil. When a complex 

 natural organic material is added to the soil, it is acted upon by various 

 groups of microorganisms, including the fungi, actinomyces and bac- 

 teria. There is no doubt that some of the protozoa and the various 

 worms and insects present in the soil ingest the organic materials, utiliz- 

 ing their various constituents as nutrients thereby causing various 

 physical and chemical changes, the extent of which depends upon the 

 nature of the organism and the environmental conditions. As a result 

 of these activities, a number of the constituents of the organic tissues 

 are broken down by processes of hydrolysis, oxidation, reduction, and 

 condensation. These processes make available the locked up energy, 

 which is utilized by the microorganisms for their activities of mineraliz- 

 ing the organic matter and liberating the inorganic compounds of nitro- 

 gen, phosphorus, potassium, magnesium, etc. in forms available for 

 plant growth. 



The composition of plants varies at different stages of growth. Green 

 plants are rich in soluble sugars and soluble nitrogenous compounds; 20 



19 Gardner, W. A. The decomposition of salicylic aldehyde by soil organ- 

 isms. Science N. S., 60: 503; also p. 390. 1924; Robbins, W. J. The cause of 

 the disappearance of cumarin, vanilin, pyridine and quinoline in soil. Ibid. 

 44:894-895. 1916. 



20 Bogdanov, S. The culture of buckwheat. Selsk. Khoz. i. Liesovod., 193: 

 227-271. 1899 (Exp. Sta. Reed., 11 : 724) ; Singleton, G. H. Nitrogen availability 

 studies on crops harvested at different stages of growth. N. J. Agr. Exp. Sta. 

 Bui. 421. 1925. 



