Kj;\iKws 117 



containing a high percentage of mineral salts. Raw humus is found in 

 coniferous forests of the more usual type, and such humus can often 

 be stripped from the bare mineral soil in a series of layers in various 

 stages of decomposition. 



The combinations of nitrates in the soil and their distribution has a 

 direct influence upon the productive capacity of the soil. Litter always 

 contains a certain amount of nitrogen, and a knowledge of the soil 

 processes through which the nitrogen is converted into an assimilable 

 form should be of great importance. This is now occupying the atten- 

 tion of a number of prominent investigators and much has already been 

 accomplished. It is known that the decomposition of humus is at- 

 tended by the liberation of ammonia, which can be quite directly assimi- 

 lated by some plants. In determining the "decay capacity" of a soil, a 

 sterilized solution of peptones is inoculated with, the soil and allowed 

 to stand for a number of days in an incubator. The extent of decay is 

 then determined by adding magnesia to the peptone solution and dis- 

 tilling off the ammonia by heating the mixture. The ammonia that is 

 evolved is conveniently collected in sulphuric acid. 



While a great many organisms can break down organic material with 

 the formation of ammonia, there are only a few, so far as is known at 

 present, that can convert the ammonia into nitrites and finally into 

 nitrates. The classes of bacteria that can accomplish this were discov- 

 ered by a Russian bacteriologist, Winogradsky, in the early nineties of 

 the past century. The first class, the nitri-bacteria, attack the ammonia 

 compounds, transforming these into nitrites. The second class, the 

 nitro-bacteria, work on the nitrites, the end products being assimilable 

 nitrates. As the nitri-bacteria can work only on ammonium com- 

 pounds, the formation of ammonia appears to be a necessary phase in 

 the development of nitrates in humus soil. Other denitrifying bacteria 

 arc usually found in hunuis soil, but under normal circumstances the 

 nitri- and nitro-bacteria are much more active. 



According to i-Iesselman. the sources from which the nitrogen forest 

 soils is derived are as follows : 



A. Decomposing litter. The nitrogen in this way is augmented by 

 that obtained from the atmosphere and fixed by legumes, alder trees, 

 and other plants. 



B. Fungi and bacteria in the soil which have the power of assimilat- 

 ing nitrogen and which l)reak up the organic compounds in the soil to 

 obtain needed energy for this process of assimilation. 



C. Ammonia and nitric acid precipitated from the atmosphere by 

 rain-water. 



