The Soil. 59 



The oxidation of humus not only makes the nitrogen, which 

 it contains, available to plants, but it also liberates the ash con- 

 stituents combined with the humus and enables them to take part 

 again in the nourishment of the growing crop. 



Oxidation is most active in soils under tillage. In arable land 

 the production of available plant food is at its maximum and 

 so is also the waste by drainage. The nitrogenous humus matter 

 of tilled land is maintained only when the new supply from crop 

 residues and organic manures is equal to the amount annually 

 oxidized. In an untilled pasture or forest soil, on the other hand, 

 a considerable accumulation of organic matter may take place, 

 the annual residue of dead leaves and roots being often in excess 

 of the amount oxidized. 



In a peat bog oxidation is further checked by the high water 

 level, which excludes air from the soil ; under such conditions an 

 unlimited accumulation of organic matter may take place if 

 plants capable of growing under these circumstances are present. 



Dentrification TVhen a soil is not in an aerated condition, 

 but has the spaces between the particles filled with water, the 

 nitrates present are destroyed by certain kinds of bacteria, the 

 oxygen of the nitrate combining with carbon to form carbon 

 dioxide, while the nitrogen is set free and returned to the air 

 in its elemental condition. If a soil be consolidated, water- 

 logged or highly charged with oxidizable carbonaceous matter, 

 the conditions become favorable for denitrification. Conditions- 

 favorable to nitrification, such as a plentiful supply of oxygen 

 and absence of acidity, are those unfavorable to denitrification, 

 so that the farmer in producing proper conditions for the former 

 desirable process is at the same time preventing the injurious 

 denitrification. The application of very large dressings of 

 manure, along with nitrate of soda, sometimes causes a consid- 

 erable loss of nitrogen from this process of denitrification. 



Fixation of atmospheric nitrogen in soils. Besides the organ- 

 isms associated with leguminous plants and which assimilate 

 atmospheric nitrogen freely when in union with the roots of thi) 



