324 CROP ROTATION 



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 growing plant 

 feeds. The quantity of this found in the soil at different periods 

 under different plants has been measured at the Utah Experiment 

 Station and the average results for twelve years are given in tabular 

 form below, stated as pounds of nitric nitrogen per acre to a depth 

 of six feet. 



Season. 



Crop. Average. 



Spring. Midsummer. Fall. 



Alfalfa 22.3 15.8 32.8 23.6 



Oats . 35.7 14.1 20.6 23.5 



Corn . . 24.8 18.9 22.0 21.9 



Potatoes 81.1 60.8 54.2 65.3 



Fallow 81.5 53.6 62.6 65.9 



The legume, alfalfa, removes the nitric nitrogen from the soil 

 equally as fast as do the non-legumes. 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 does 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 a lack of its formation, 

 as it is not needed by the legume, and hence not formed. This 

 conclusion, however, 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. 



Milligrams, nitric nitrogen produced 

 in 100 gms. of soil in twenty-one days. 



Crop. > Average. 



Spring. Midsummer. Fall. 



Alfalfa 3.15 7.48 3.08 4.56 



Oats 2.40 4.00 3.00 3.13 



Corn 2.18 3.50 1.48 2.38 



Potatoes . . .... 3.00 15.55 5.60 8.04 



Fallow 1.30 5.50 2.48 3.09 



Here the quantity of soluble nitrogen produced in the alfalfa 

 soil is. greater than that produced in either the oat, corn, or fallow 

 soil. There is no doubt that this is one reason why an increased 

 yield is obtained the year following the plowing up of legumes for this 

 increased action also occurs the next year after an alfalfa field is 

 planted to some other crop. This is due to the stimulation of 

 bacterial organisms of the soil by the alfalfa plant so that they 

 make available faster the nitrogen of the soil, but this only depletes 

 the soil of its nitrogen more readily than the non legume, as it is 

 the nitrogen already combined in the soil on which the nitrifying 



