152 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



In the water-logged condition, rice soils present conditions 

 favorable for nitrate reduction. If nitrate is added to such soils 

 as fertilizing material it undergoes reduction to nitrite and gaseous 

 nitrogen. Either of these transformations is quite undesirable. 

 The nitrite is toxic to plant growth, and gaseous nitrogen is no 

 longer available to the plants. Ammonium salts are more suit- 

 able as nitrogenous fertilizers under such cultural conditions. 



Summary. — In soil, nitrogen undergoes a series of transforma- 

 tions as a result of the activities of microorganisms. The supply of 

 combined nitrogen in the soil has been furnished to a considerable 

 extent through the activities of certain groups of microorganisms 

 capable of utilizing the gaseous elementary nitrogen of the atmos- 

 phere for the purpose of building up complex organic substances, 

 namely, the constituents of the microbial cells. These organisms 

 are either non-symbiotic in nature, leading a free existence in the 

 soil, or are symbiotic, namely, growing in the cells of higher plants, 

 as in the roots of legumes. When these bacteria die, the higher 

 plants obtain the nitrogen fixed by these organisms, either directly 

 through the action of plant enzymes, or indirectly through the 

 activities of microbes which bring about the decomposition of the 

 complexes built up by the nitrogen-fixing organisms. 



When the nitrogenous plant residues are added to the soil, in the 

 form of roots, leaves, twigs, green manures, and stable manures, 

 they first undergo a series of decomposition processes carried out 

 by various groups of microbes with the result that the nitrogen is 

 changed from complex organic substances into a simple inorganic 

 form (ammonia). The processes of decomposition are accom- 

 panied by processes of synthesis converting again the simple forms 

 of nitrogen into complex organic forms of a microbial nature. 

 The nitrogen content of the plant is apt to be very low, ranging 

 from 0.2 per cent in certain cereal straws to 3 per cent in certain 

 leguminous plants. The first result of the decomposition of plant 

 residues by microbes is a narrowing of the ratio of carbon to nitro- 

 gen in the decomposed material or increasing the percentage 

 nitrogen content. 



If the amount of nitrogen in the plant residues is sufficiently 

 large, as 1.7 per cent or more, decomposition proceeds rapidly, and 

 the liberation of nitrogen in such an available form as ammonia 

 takes place within 4 to 6 weeks of decomposition under favorable 

 conditions. If the relative nitrogen content is low, considerable 



