TILLAGE 665 



paragraphs of this chapter the effect of the commoner 

 types of tillage implements on the soil are pointed out 

 as a guide to their selection for the accomplishment of 

 a desired modification. Good soil management consists, 

 first, in analyzing the soil conditions, in order to deter- 

 mine the change that should be effected ; and second, in 

 the selection of the implement or other treatment that 

 will most readily and economically accomplish the object. 



568. Classes of tillage implements. According to 

 their mode of action, tillage implements may be divided 

 into three groups, plows, cultivators, packers and 

 crushers. 



569. 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 condi- 

 tion for plowing, it will be shattered and broken up; 

 (2) the soil is partially or wholly inverted, and any rubbish 

 is put beneath the surface. 



570. Pulverizing action of the plow. In twisting, the 

 soil tends to shear into thin layers, as already pointed 

 out (par. 128). These layers are moved unequally upon 

 each other, as the leaves of a book when they are bent. 

 The result should be a very complete breaking-up of the 

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

 on (1) the condition of the soil, and (2) the type of plow. 

 As to the condition of the soil, there is a certain optimum 

 moisture content at which the best results will be obtained. 

 That condition of moisture is the one that is best for 

 plant growth. Any departure from this optimum moisture 

 content will result in less efficient work. It has been said 

 that, in proportion to the energy required, the plow is 

 the most efficient pulverizing implement used by the 



