SOIL STRUCTURE AND ITS MODIFICATION 145 



purposes: (1) modification of the structure of the soil; (2) 

 disposal of rubbish or other coarse material on the surface, and 

 the incorporation of manures and fertilizers into the soil ; and 

 (3) the deposition of seeds and plants in the soil in position for 

 growth. 



The most prominent of these purposes is the modification 

 of the soil structure. This affects the retention and movement 

 of moisture and air, the absorption and retention of heat, 

 and either promotes or retards the growth of organisms. 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 may prevent 

 the evaporation of moisture. In fine-textured soils, in which 



1 2 5 



Fig. 25. — Three types of plow bottoms; 1, stubble; 2, sod; 3, general 



purpose. 



the granular structure is most desired, tillage may have an 

 important influence on the formation or destruction of gran- 

 ules. As has been pointed out, any treatment that increases 

 the number of lines of weakness in the soil structure facili- 

 tates the activities of the moisture films and the colloidal mate- 

 rials in producing soil granules. Tillage shatters the soil and 

 breaks it into many small aggregates, which may be drawn 

 together and loosely cemented as a result of the evaporation 

 of moisture. The more numerous the lines of weakness pro- 

 duced, the more pronounced is the granulation; and, con- 

 versely, the fewer the lines of weakness produced, the more 

 coarse and cloddy is the structure. 



According to their mode of action, tillage implements may 



Kamsower, H. 0., Equipment for the Farm and Farmstead; Boston, 

 1917. 

 King, F. H., Physics of Agriculture; Chap. XI, Madison, Wis., 1910. 



