156 SOILS AND MANUEES 



The production of ammonia from the proteids of anima] 

 and vegetable matter in the soil is probably effected, 

 directly and indirectly, by the action of moulds, as well 

 as by many different types of bacteria. An organism 

 known as bacillus mycoides, usually very abundant in 

 arable soils, appears to be particularly active, and is com- 

 monly credited with a large share of the work. It has been 

 definitely ascertained that this microbe causes the decom- 

 position both of proteids and amides, such as those 

 mentioned above, with the production of ammonia. 



Production of Nitric Acid from Ammonia. The con- 

 version of ammonia into nitric acid is due exclusively to 

 the action of bacteria. It takes place in two distinct steps. 

 First the ammonia is oxidised to nitrous acid, and then the 

 latter is further oxidised to nitric acid. Each of these two 

 steps is effected by a separate and particular type of 

 organism, neither of which can perform the work of the 

 other. According to Winogradsky, the nitrifying organisms 

 obtained from soils in widely different localities are not 

 identical, but each soil contains only one type of organism 

 capable of oxidising ammonia to nitrous acid, and one 

 capable of oxidising nitrous acid to nitric acid. 



The Nitric and Nitrous Ferments. The two kinds of 

 microbe are quite distinct, and, when isolated, are easily 

 distinguished by their appearance under the microscope. 

 The nitrous ferment (Fig. 23) 1 is described as consisting 

 of nearly spherical corpuscles, i.e., micrococci, of about 

 a thousandth part of a millimetre diameter, and fairly 

 uniform both in size and shape. The nitric ferment 

 (Fig. 24) l is of a more cylindrical form, one dimension 

 being two or three times greater than another, and is, 

 therefore, generally classed with the bacilli. It is smaller 

 than the nitrous ferment, its length being only about half 

 the diameter of the latter. 



1 Winogradsky, Annales de 1'Institut Pasteur, 1891. 



