12 T. Lyttleton Lyon and James A. Bizzell 



occurrence of nitrates in the soil on which they grow. For any one soil 

 the nitrate content is largely dependent on the kind of crop groxsTi. Fur- 

 thermore, the stage of growth of any particular plant has a characteristic 

 relation to the nitrate content of the soil. The extensive literature bearing 

 on the nitrate content of soils contains a number of instances illustrating 

 this relationship between the soil and the crop growing on it, and these 

 may be so classified as to bring out definite relations. 



Experiments showing different nitrate contents of soils under 



different crops 



In an extensive series of field experiments in which nitrates were deter- 

 mined m samples of soil taken under several crops, King and Whitson 

 (1901) found that, for the four crops named, nitrates were highest 

 during the growing season under maize, next under potatoes, and lowest 

 under alfalfa and clover. 



Stewart and Greaves (1912), in another extensive set of experiments 

 extending over several years, found the nitrate content under maize, 

 potatoes, oats, and alfalfa to rank in the order named. 



In field soil planted to different crops Wollny (1897) found that the 

 quantities of nitric nitrogen, expressed in kilograms per German acre, 

 were 9.2 under beans, 22 in fallow, and 1.2 under wheat. 



Brown (1912) sampled soil of neighboring plats that continuously for 

 several years had gro\\Ti maize and clover, respectively, and found that 

 the processes of ammonification and nitrification were more rapid in the 

 soil planted to maize. 



Voorhees, Lipman, and Brown (1907) inoculated a modified Omeli- 

 anski's solution with soil that had grown oats, soil that had grown crimson 

 clover, and soil that remained unplanted during the period required 

 to mature the crops. The resulting formation of nitrates was greater in 

 the solutions inoculated with, the unplanted soil than in those inoculated 

 with the soil planted to either of the crops. The quantity of nitrates 

 found, however, was small. An ammonification test resulted in a greater 

 production of ammonia from the planted than from the unplanted soil. 



Brown and Maclntire (1910) found the average nitrate content for 

 the grownng season to be: under maize, 55.5 parts per million of the dry 

 weight of the soil; under oats, 18 parts per million; under wheat, 7.9 

 parts per million; imder grass, 1.4 part per million. The average moisture 



