70 CULTIVATION AND TILLAGE 



by making the soil warmer, and by aiding in hastening the 

 chemical processes. 



Denitrification is the re\'erse of nitrification, and is the 

 result of the working of a class of bacteria that break down 

 the nitrates, setting free nitrogen, which passes off as a gas. 

 The condition necessary for this class of organisms to work 

 is the absence of air, and when frequent thorough culti^a- 

 tion is given and the soil is well aerated these bacteria 

 perish. So it follows that the best way of eradicating these 

 injurious bacteria is to maintain thorough and complete 

 cultivation. Besides destroying the denitrifying bacteria 

 more plant food is made available by increasing and by 

 stimulating the reproduction and the growth of the friendly 

 nitrifying organisms. 



Fig. 22. — A single horse adjustable cultivator. 



Implements for Cultivation. — ^The implements for cultiva- 

 tion may briefly be summed up as follows: (1) The plows, 

 of which there are the walking plow, the sulky plow, the 

 gang plow, the disk plow, and the steam plow; (2) the 

 harrows, of which there are the smoothing harrow, the 

 spring-tooth harrow, the disk harrow, the cut away disk 

 harrow, the spading harrow, and the plow cut-disk harrow; 



