42 SOILS AND PLANT LIFE 



depends also upon the size of the weeds and the nature 

 of their root system. Any plant is most easily destroyed 

 when it is just breaking through the ground. The com- 

 mon harrow is one of the best tools we have for destroying 

 young weeds. 



As the weeds become a little older, the disc or cultiva- 

 tor is used to destroy them. The disc is not used, however, 

 on such plants as the quack grass. This weed is spread 

 by long, underground stems, which run about beneath the 

 surface at a depth of two or three inches. A disc only 

 serves to cut these stems into pieces and scatter them 

 through the fields, where each piece becomes a new plant. 

 Such weeds are plowed up, exposing the roots to the sun 

 without breaking them any more than necessary. 



30. Forming a Dust Mulch. — In Section 14, we ob- 

 served how the moisture rises through the soil until it 

 escapes into the air unless prevented by a dust mulch. 

 Such a mulch may be formed and maintained with the 

 harrow, the fine-toothed cultivator, or with the subsurface 

 packer, which is extensively used in the dry farming regions. 



31. Making it Possible for Air and Water to enter the 

 Soil. — Plants can not exist without air and water. The 

 roots and germinating seeds, as well as the leaves and 

 stems, must have air. Plants absorb no moisture through 

 their leaves or stems; the roots alone gather the water. 

 We have already seen in Section 16 why the soil needs air. 

 It must be put in condition to receive and hold this air 

 and water. 



The plow stirs the soil, making it light and friable; 

 the harrow breaks the lumps, making the earth even more 

 mellow; and the cultivator loosens the surface so that 

 the rain will not run off and that the air may enter. 



