

BACTERIA AND SOIL FERTILITY 



soils when they contain about 60 per cent of the water which the 

 soil will hold when saturated. 



Work at the Utah Experiment Station demonstrated that the 

 application of irrigation water to a soil has a distinctly beneficial 

 effect upon nitrification. It is greatest where 15 inches of water 

 were applied yearly. Under this condition the nitric nitrogen pro- 

 duced was 28.5 pounds per acre- foot of soil. The greatest 

 benefit per inch of water was obtained when only 7.5 inches of 

 water was applied. Excessive quantities of water retarded nitrifi- 

 cation and leached the nitrates beyond the roots of the growing 

 plants. 



Food. — The nitrifying bacteria are similar to other plants in 

 that they can live on inorganic salts. Even carbon dioxid sufiices 

 as their source of carbon. The green plants with few exceptions 

 are the only other living organisms known to use carbon dioxid. 

 These get the energy necessary to do this thru the action of their 

 chlorophyll on sunlight. The fuel which the nitrifying ferments 

 burn in their tiny engines is ammonia in the case of nitrosomonas 

 and nitrites in the case of the nitrobacter. The ashes from this 

 quiet but interesting combustion are nitrates. Workers have 

 traced a definite relationship between the amount of ammonia 

 oxidized and the carbon assimilated, as seen from the following: 





Experiment 





I 



2 



3 



4 





mg. 



722.0 

 19.7 



36.6 



mg. 



506.1 



15.2 



22-3 



mg. 



928.3 



26,4 



3S''^ 





Ammonia oxidized (expressed as nitrogen) 

 Carbon assimilated 



Ratio — Nitrogen : Carbon 



815.4 

 22.4 



36.4 



Hence 70 to 80 pounds of nitrogen are oxidized in soil by these 

 organisms for every 2 pounds of carbon they assimilate. 



When growing in solutions these organisms are injured by 



