178 



BACTERIA AND SOIL FERTILITY 



Experiments conducted at the Utah Experiment Station during 

 twelve years have demonstrated that even on soils poor in nitro- 

 gen the legume first feeds upon the combined nitrogen of the 

 soil. We have seen that plant residues and other complex nitrogen 

 compounds found in the soil are transformed by bacteria into 

 ammonia, and this in turn by another class of bacteria into nitric 

 nitrogen, and it is mainly on this nitrogen that the grov^^ing plant 

 feeds. The quantity of this occurring in the soil at different 

 periods under different plants, as measured at the Utah Experi- 

 ment Station, and the average for twelve years, are given in 

 tabular form below. These results are stated as pounds of nitric 

 nitrogen per acre-foot of soil to a depth of six feet. 



Crop 



Season 



Average 



Spring 



Midsummer 



Fall 



Alfalfa 



Oats 



Corn 



Potatoes 



Fallow 



22.3 



357 

 24.8 



81. 1 



81.5 



14.1 

 18.9 

 60.8 



S3^^ 



32.8 

 20.6 



22.0 



54-^ 

 62.6 



23,6 



21.9 



H'3 

 65-9 



The legume removes the nitric nitrogen from the soil equally 

 as fast as does the non-legume. Yet this soil was well inoculated 

 with the symbiotic bacteria which undoubtedly assisted the alfalfa 

 in obtaining free nitrogen from the air when needed, but not 

 until the soluble nitrogen had been drained from the soil to its 

 full extent, as shown by the fact that alfalfa soil never contains 

 more than oat and corn land and is very poor as compared with 

 potato and fallow soil. 



Nitrification in Soils. — ^It may be argued that the small 

 quantity of nitric nitrogen in the alfalfa soil is due to low nitrifi- 

 cation as nitrates are not needed by the legume and hence not 

 produced. This conclusion is not warranted by the facts in the 

 case, as may be seen from the results obtained where nitrification 

 was measured. These also are the average results extending over 

 a number of years and obtained at the Utah Experiment Station. 



