OF THE PRECEDING CHAPTERS. 179 



Injuries arising from wetness of the soil. It causes 

 the formation of vegetable acids, and other hurtful sub 

 stances. 



These defects to be removed by draining. 



Drains to be 30 to 36 inches deep, and always co 

 vered. If made of stones, these should be broken 

 small; if k of tiles, these may be either of the round, 

 oval, or horseshoe shape. The earth to be rammed 

 hard above them in all cases. They ought to run 

 straight down slopes, and be placed 24 to 50 feet 

 apart. 



Subsoil and trench ploughing; difference in the two 

 operations, and nature of their effect. 



The inorganic substances of the soil are found in 

 plants, with the single exception of alumina. 



The quantity of some of them is quite small in 

 plants, but all are absolutely necessary. 



CHAPTER VII. 



Effect of cropping upon the soil. 



Different crops take away the inorganic substances 

 of the soil in different proportions; their ash also va 

 ries in composition. 



The grains contain chiefly phosphates. 



Potatoes and turnips, mostly potash and soda. 



Grasses, for the most part, lime and silica; straws, 

 nearly all silica. 



This explains the principle of rotation. One crop 

 may find food when the land has been exhausted for 

 another, and so a succession may be continued for 

 some years. 



The value of land is kept up by such a course for a 

 greatly increased length of time. 



