I4O SOILS AND FERTILIZERS 



below where the sunlight is excluded by the surface 

 soil it is most active. In a corn field it is more active 

 than in a compacted fallow field. 



154. Base -forming Elements Essential for Nitrification. 



- The presence of some base-forming element to com- 

 bine with the nitric acid produced is a necessary con- 

 dition for nitrification, and for this purpose calcium 

 carbonate and sodium and potassium compounds are 

 particularly valuable. The absence of alkaline salts is 

 one of the frequent causes of non-nitrification. *In acid 

 soils the process is checked for the want of proper 

 basic material. The organisms which carry on the 

 work cannot exist where there are strong acids or 

 alkalies, consequently in such soils nitrification cannot 

 take place. 



* 



155. Nitrous Acid Organisms. There are at least 

 two nitrifying organisms in the soil : one produces 

 nitrates and the other nitrites or nitrous acid. It has 

 been shown that the nitrification process takes place in 

 two stages : first nitrites are produced by the nitrous 

 organism, and then the process is completed by the 

 nitric organism. Warington says that " both organ- 

 isms are present in the soil in enormous numbers, and 

 the action of the two organisms proceeds together, as 

 the conditions are favorable to both." As a result 

 of the workings of the nitrous acid organism, nitrites 

 are formed, and these are acted upon by the nitric acid 



