252 SOILS AND FERTILIZERS 



year before the weeds become firmly established. 

 Weeds are most easily destroyed at the time of germ- 

 ination and before the leaves appear above ground. 

 The plow should be relied upon largely for the de- 

 struction of deep rooted perennial weeds, while the 

 cultivator is effectual for the destruction of annuals. 

 When weeds are plowed under or destroyed by culti- 

 .vation they add vegetable matter and humus to the 

 soil and thus are made to improve the condition of 

 the soil 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. 



338. Influence of Cultivation Upon Bacterial 

 Action. The cultivation of the soil has a marked 

 influence upon bacterial action. Some of the soil 

 organisms as the nitrifying organisms, (See Section 

 139) require oxygen for their existence, hence culti- 

 vation which increases the supply of oxygen in the 

 soil increases the activity of such organisms. In acid 

 peaty soils, aeration induces bacterial action which re- 

 sults in more rapid decay and a lowering of the. per 

 cent, of total organic matter including the deleterious 

 organic acids. The neutralizing of the organic acids 

 of soils by applications of lime and wood ashes hastens 

 bacterial action. During the process of nitrifica- 

 tion, the bacterial action is not alone confined to the 

 nitrogenous compounds of the soil, the nitrifying 

 organisms require phosphates as food which are left 



