OPERATION OF TILLAGE IMPLEMENTS 469 



the soil: (1) Those which loosen the soil structure. 

 (2) Those which compact the soil structure. In the 

 subsequent paragraphs of this chapter the effect of the 

 more common types of tillage implements on the soil 

 are pointed out as a guide to their selection for the ac- 

 complishment of a particular desired modification. 

 For, good soil management consists, first, in analyzing 

 the soil conditions, to determine the change which 

 should be effected; second, in the selection of the im- 

 plement or other treatment which will most readily 

 and economically accomplish the object. 



335. Mode of action. According to their mode of 

 action, tillage implements may be divided into three 

 groups: (a) Plows. (6) Cultivators, (c) Crushers and 

 packers. 



336. Plows. The primary function of a plow is to 

 take up a ribbon of soil, twist it upon itself, and lay 

 it down again bottom side up, or partially so. In the 

 process two things result. (1) If the soil is in proper 

 condition for plowing, it will be shattered and broken up. 

 (2) The soil is inverted, and any rubbish is put beneath 

 the surface. 



337. Pulverizations. In twisting, the soil tends to 

 shear into thin layers, as pointed out by King. These 

 layers are moved unequally upon each other, as, when 

 the leaves of a book are bent, they slip past each other. 

 The result should be a very complete breaking up of 

 the soil. How thorough the breaking-up will be will 

 depend upon (a) the condition of the soil, and (b) the 

 type of plow. As to the condition of the soil, there is 

 a certain optimum moisture content at which the best 



