124 



SOILS. 



nated in the form of water and carbonic dioxid, while at the 

 same time there is an increase in the percentage of carbon, and 

 generally also of nitrogen ; the latter more particularly in the 

 case of vegetable matter not very rich in that element. When 

 once humification is complete, oxidation, especially under arid 

 conditions, bears mainly upon the carbon and hydrogen, so 

 that the nitrogen content may rise to very high figures ; while 

 another portion is ultimately wholly oxidized, with the forma- 

 tion of nitrates, under the influence of the nitrifying bacteria, 

 this being the process chiefly efficient in the nutrition of vegeta- 

 tion with nitrogen. 



As a matter of course, the several organic compounds contained in. 

 plants may continue to exist in soils for some time, varying according 

 to conditions of temperature and moisture. Thus dextrin, glucose, and 

 even lecithin and nuclein have been reported to be found. The activity 

 of the numerous fungous and bacterial ferments under favoring condi- 

 tions will, of course, limit the continued existence of such compounds 

 somewhat narrowly, so that they can hardly be considered as active soil 

 ingredients save in so far as they favor the development of the bacterial 

 flora. 



Porosity of Humus. One of the essential' features of na- 

 tural humus is its great porosity, whereby it not only becomes 

 highly absorbent of water and gases, but is also gradually oxi- 

 dized, probably under the influence of bacteria. For this oxi- 

 dation, as measured by the evolution of carbonic gas, pro- 

 gress's most rapidly under the same conditions as to moisture, 

 temperature and access of air, that are known to be most favor- 

 able to fungous and bacterial growth. 1 I nice the formation of 

 carbonic dioxid in the soil is assumed to be the measure of the 

 intensity of such activity. 



Physical ami Chemical Xatnrc of the Humus Substances. 

 The humus substances are gelatinous when moist, but are 

 neither markedly adhesive or plastic. Like the other colloidal 

 substances of the soil, they serve to retain both gases and 

 vapors, including moisture, liquid water, and its dissolved 

 solids. In the natural, porous condition they are powerfully 

 absorbent of gases, including especially aqueous vapor. Dry 

 humus swells up visibly when wetted, the volume-weight in- 



