WEEDS AND THEIR CONTROL 489 



345. Clod-crushers. The aim of these implements 

 is to break up lumps. As to mode of action, there are 

 several forms. The bar roller and the "clod-crusher" 

 (see Fig. 71) concentrate their weight at a few points, 

 and are open enough so that the fine earth is forced up 

 between the bearing surfaces. They are very effective 

 in reducing lumpy soil to comparatively fine tilth. They 

 have very little leveling effect further than the breaking 

 down of lumps. 



The planker, drag or float, variously so-called, con- 

 sists essentially of a broad, heavy weight without teeth, 

 which is dragged over the 

 soil. The lumps are rolled 

 under its edge and ground 

 together in a manner which 

 very effectively reduces their fig. 149. Float or smoother made 

 size. At the same time, the of P lanks - 



soil is leveled, smoothed, and, to a degree, compacted. 

 It may well be used in the place of the roller as a pul- 

 verizer, on many occasions. It is constructed in many 

 forms. 



III. OTHER PHASES OF TILLAGE OPERATION 



In addition to the modification of food, moisture, 

 air and heat of the soil, through changes in its structure 

 as a result of tillage and other cultural practices, other 

 important soil conditions may be changed. Two of the 

 most important of these are: (1) The destruction of 

 weeds. (2) The control of erosion. 



346. Weeds in their relation to crop-production. 

 A weed has been defined as a plant out of place. By 



