126 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



plants, nitrogen fixation appears to take place, similar to that in the 

 legumes, and advantage is taken of this fixation in India by using 

 such plants as green manuring crops. 



Decomposition of Proteins by Microorganisms. — Whether 

 the nitrogen is fixed in the soil through the agencies of nitrogen- 

 fixing bacteria, whether it is introduced there in the tissues of 

 plants, animals, and microorganisms in the numerous organic 

 residues, or whether it is added to the soil in the form of organic 

 fertilizers — it is not directly available to the growth of higher 

 plants. This nitrogen occurs to a large extent in the form of pro- 

 teins and their derivatives, and to a less extent in the form of other 

 complex organic nitrogenous compounds. Before this nitrogen 

 can be utilized for the growth of green plants, these complex 

 organic substances have to be decomposed and the nitrogen 

 changed to other forms. This process of liberation of nitrogen 

 is carried on in the soil by numerous fungi, actinomyces, bacteria, 

 and invertebrate animals, and is one of the most important activi- 

 ties of the microbes in the soil. It is also one of the processes 

 common to a large portion of the microorganisms inhabiting the 

 soil. 



Proteins, the most important group of nitrogenous compounds 

 in the living organism, are made up of chains of certain units or 

 building stones, namely, the amino acids. So far, between 20 and 

 30 different amino acids have been isolated from the plant and 

 animal organism and described. These amino acids can be com- 

 bined in various ways, giving a number of complex bodies, which 

 are typical of the proteins and their derivatives found in the living 

 and dead animals, plants, and microbes. Living processes involve 

 the building up of new tissue substance or cell material, of which 

 the proteins are vital constituents; among the processes associated 

 with the decomposition of the dead bodies of plants and animals 

 is the decomposition of these proteins into simpler compounds. 



Some of the amino acids are comparatively simple in compo- 

 sition; others are more complex, as shown by some typical 

 formulae : 



Glycocoll or glycine . CH2(NH2) • COOH 



Aspartic acid CH2 • COOH -CHCNHg)- COOH 



Arginine (NH)C(NH2) NH- (CH2)3CH(NH2) COOH 



Tyrosine C6H4(OH) • CH2 • CH(NH2) • COOH 



