158 INFLUENCE OF MANURE ON THE SOIL 



uent of the mustard plant, rnay be regarded as a derivative of 

 carbon bisulphid and probably would have a similar influence upon 

 the soil microflora. 



3. The growing of a crop during the season of the year when the 

 heavy rains would wash much of the nitric nitrogen beyond the 

 roots of the plants prevents this loss, for the nitrogen is stored in 

 the body of the plants and is later liberated by decomposition for 

 succeeding plants. 



4. The growth of the legumes may actually lead to an increase in 

 the nitrogen of the soil. The method, extent, and conditions under 

 which this occurs is considered in detail in later chapters. 



The results, however, following the use of green manures are not 

 uniformly beneficial, for the following ill effects have at times 

 followed its use: 



1. The physical condition of the soil may be injured. It becomes 

 too loose and open, decomposition being thereby decreased and 

 leaching increased. This was probably the reason why Brown did 

 not always find an increase in the bacterial activities following the 

 application of green manure. This is also true with regard to the 

 work reported by Laurent. 



2. Engberding, studying the effect of straw and sugar upon the 

 number of bacteria in the soil, found at first an increase, followed by 

 a decrease. The ammonifying and nitrogen-fixing groups of bacteria 

 showed an increase, but the nitrifying group was retarded. 



Fischer, in his paper on the changes undergone by nitrogen in 

 sandy and clayey soils, offers an explanation for the loss of nitrates 

 in a soil to which carbohydrates have been added in the following 

 reaction : 



24 NaNOs + 5 r 6 Hi 2 O 6 = 24 NaOH + 12 N 2 + 30 CO 2 + 18 H 2 O 



The oxygen of the nitrate is used by the ferments for the oxidation 

 of the carbohydrate and the nitrogen is liberated as a gas. 



Frankfurt and Duschechkin state that green manure under field 

 conditions caused a diminution of the nitrate content. Both legumes 

 showed this effect, but they consider it as due to the action of the 

 manure upon the soil moisture. . 



Stevens and Withers give two reasons why the activity of nitrify- 

 ing organisms, in pure culture under laboratory conditions, cannot be 

 compared with their activities under conditions in the field: (1) "In 

 mixed culture their symbiotic and physiologic relations are so 

 different from those obtained in pure cultures that their metabolic 

 processes are with difficulty expressed;" (2) "the presence of large 

 amounts of solid matter, sand, or earth, in contact with the liquid 

 medium, so alters its relation to the nitrifying organisms that their 

 physiologic activities and metabolic products are different." These 

 authors say in conclusion: "In the light of the facts set forth, the 



