298 SOILS AND FERTILIZERS 



relied upon largely for the destruction of deep-rooted 

 perennial weeds, while the cultivator is effectual for 

 the destruction of annuals. When weeds are plowed 

 under or destroyed by cultivation they serve as a green 

 manurial crop, adding vegetable matter and humus to 

 the soil and thus improving its condition instead of 

 reducing the yield of crops by appropriating fertility, 

 as they do if allowed to grow and mature. Cultivation 

 which secures aeration of the soil and conservation of 

 the soil moisture is also effectual for the destruction 

 of weeds. 



353. Influence of Cultivation upon Bacterial Action. - 



Cultivation has a marked influence upon bacterial ac- 

 tion. Some of the soil organisms, as the nitrifying 

 organisms (see Section 150), require oxygen for their 

 existence, hence cultivation which increases the sup- 

 ply of oxygen in the soil increases the activity of such 

 organisms. In the absence of air, anaerobic fermenta- 

 tion occurs, and such fermentation is unfavorable to 

 crop growth. When acid peaty soils are aerated bac- 

 terial action is induced which results in more rapid 

 decay and a lowering of the per cent of total organic 

 matter, including the deleterious organic acids. Neu- 

 tralizing the organic acids of soils by applications of 

 lime and wood ashes hastens bacterial action, and during 

 the process of nitrification this is not alone confined 

 to the nitrogenous compounds of the soil, as the nitri- 

 fying organisms require phosphates as food and these 



