236 Physics of the Soil 



row represented in Fig. 72 does good work. Its weight 

 forces it into the soil and the elasticity of the teeth prevent 

 them from being broken, but such tools can never do the 

 degree of pulverizing that the disc harrow accomplishes. 



289. Smoothing Harrows. When the soil has been pul- 

 verized with the disc or other tool and it is desired to leave 

 the surface more nearly even, or where the soil is naturally 

 very mellow, making less force necessary to change the 

 surface texture, then the heavier weights of tilting har- 

 rows, Fig. 73, may be used to great advantage on account 

 of the greater area which may be covered with them in a 

 day and their lighter draft 



FIG. 74. The planker. 



290. The Planker. It is sometimes desirable to leave the 

 surface particularly smooth without firming it and at the 

 same time to crush lumps. This may be done by means of 

 a planker made of three to five 8- or 10-inch plank 

 bolted together with their edges overlapping as represented 

 in Fig. 74. The tool is best made of oak plank two inches 

 thick and eight to twelve feet long. Such a tool cannot 

 take the place of a roller where it is desired to firm the 

 ground. 



291. The Use of the Roller. The roller is used chiefly 

 when it is desired to firm the surf ace and to help cover seed, 

 especially when sown broadcast. In other cases it may be 

 used to crush clods or to compress the furrow slices after 

 the sod plow. Again when a green crop like rye or clover 

 has been turned under for manure, or where coarse litter 

 has been plowed under, a roller is needed to compress the 

 soil and establish good capillary connection with the deeper 

 soil water. It is sometimes used to develop a mulch where 

 grain is rolled after it is up. 



