136 SOILS AND FERTILIZERS 



them into other substances. There are three different pro- 

 cesses and three distinct products that may result. These 

 are : (1) reduction of nitrates to ammonia ; (2) reduction 

 of nitrates to free nitrogen ; (3) conversion of nitrates into 

 organic nitrogenous substances. All of these fermentations 

 result in a conversion of the more easily available forms of 

 nitrogen into less available, and in the case of the production 

 of free nitrogen there is a loss of nitrogen from the soil, as 

 the free nitrogen is a gas and passes off into the air. 



Most of the bacteria that effect these changes do so only 

 when there is a limited supply of air, so that a thorough aera- 

 tion of the soil practically prevents denitrification. Straw 

 apparently induces denitrification when conditions are at 

 all favorable for that process. 



The addition of a nitrate fertilizer to a well-drained soil 

 receiving farm manure is not likely to result in a loss of 

 nitrates unless the dressing of manure has been extremely 

 heavy. At the Rothamsted Experiment Station, where 

 large quantities of nitrate of soda are used every year in 

 connections with annual dressings of farm manure, the nitrate 

 produces nearly as large an increase when applied to the 

 manured as when added to the unmanured plat. 



Very heavy applications of farm manure, of fifty tons to 

 the acre or more, may temporarily interfere with formation 

 of nitrates. The plowing under of large quantities of straw 

 and even, under some conditions, green-manures may have 

 this effect. 



175. Nitrogen fixation. — Another and very important 

 bacteriological process is the transfer of nitrogen from the 

 atmosphere to the soil. This process is termed " nitrogen 

 fixation " and it may occur either with the assistance of higher 

 plants, or without. The first of these is called nitrogen 

 fixation through symbiosis with higher plants, the second 

 nitrogen fixation by soil organisms not associated with plants. 



