64 AGRICULTURE. [CH. 



which are employed as green manures ; but the last 

 three are as a rule too tempting as food to be entirely 

 ploughed into the land, although a portion of such 

 crops is often left on the ground for this purpose. 



124. Green manures have a mechanical action 

 on the land, rendering it more open, and therefore 

 better prepared for the roots of plants to penetrate, 

 and seek nourishment for the growing crop. 



CHAPTER VII. 

 TILLAGE OPERATIONS. 



125. These tillage operations very greatly con- 

 tribute to the productiveness of any soil. They 

 chiefly consist of ploughing, stirring, crushing, 

 and harrowing, and it will be seen that each of 

 these contributes to the required result by two distinct 

 means 



By the greater freedom with which plants are enabled 



to seek for and obtain their food; and 

 By increasing the plant food in the soil. 



The operation of ploughing brings up from beneath 

 the surface, soil which has been buried, and thereby 

 exposes fresh material to the atmosphere. Ploughing 

 is usually limited to a turning over of the soil 

 which has been previously under cultivation. When 

 the subsoil is ploughed, it is distinguished as subsoil 

 ploughing, but this is generally stirred and not 

 brought up to the surface, because it frequently has 

 harsh and acrid matter present. As a rule, very great 

 caution is necessary in bringing up to the surface any 

 of the subsoil, especially when it is at all of a sour 

 nature. There must be some good reasons for these 

 practices, and it will be well to search them out. 



126. The first effect of ploughing is to give the 



