664 SOILS: PROPERTIES AND MANAGEMENT 



growth of organisms. Through all these factors the com- 

 position of the soil solution, and finally the penetration 

 of plant roots, is influenced. The creation of a soil 

 mulch is merely a change in the structure of the soil at 

 such times and in such a manner as will prevent evapora- 

 tion of moisture. For this reason it is essential to under- 

 stand the relation of soil structure to the movement of 

 moisture in managing the mulch. In fine-textured soils, 

 in which the granular or crumb structure is most desired, 

 tillage may have an important influence on the formation 

 or destruction of granules. As has been pointed out, 

 any treatment that increases the number of lines of weak- 

 ness in the soil structure facilitates the action of the mois- 

 ture films and the colloidal material in solidifying the 

 soil granules. Tillage shatters the soil and breaks it into 

 many small aggregates which may be further drawn 

 together and loosely cemented as a result of the evapo- 

 ration of moisture. The more numerous the lines of 

 weakness produced, the more pronounced is the granu- 

 lation; and, conversely, the fewer the lines of weakness 

 produced, the more coarse and cloddy is the structure. 



566. Implements of tillage. The implements adapted 

 to the manipulation of the soil are very numerous, and 

 embrace many types. Many operations are compre- 

 hended by the term tillage, which includes the use of all 

 those implements that are used to move the soil in any 

 way in the practice of crop production. It includes the 

 smallest hand implements as well as the heaviest trac- 

 tion machinery. 



567. Effects on the soil. All these operations may 

 be divided into two groups, according to their effect on 

 the soil, those that loosen the soil structure, and those 

 that compact the soil structure. In the subsequent 



