ROTATION OF CROPS. 83 



ing in the clover-crop after it has fallen on the 

 ground, and undergone partial decay. The long tap- 

 roots of this plant, as heretofore mentioned, go down 

 very deep, and work up a great deal of material as 

 food, and when the plant decays it furnishes all its 

 elements to wheat, corn, or other crop that may fol- 

 low. Old farmers know that, if land will only pro- 

 duce clover, it can be improved. 



187. It was once thought that the advantages of 

 rotation are due in some measure to the excretion, 

 or giving out, of material by the roots of one plant 

 which serves as food to another. It is not believed 

 now that plants have the power to excrete, or give 

 out, any such material. Plants do have some power 

 of selection in taking in their food, for we know 

 that, when different plants are grown in the same 

 soil, they contain different proportions of the same 

 elements, but there is no proof that they all take in 

 the same substances and give out from their roots 

 what each does not need for growth. 



1 88. No kind of rotation will secure good crops 

 if any one or more of the elements which a crop 

 needs be entirely absent from the soil, or if the food 

 is in such a condition that plants can not appropri- 

 ate it. In such cases, fertilizing, or manuring, is the 

 only way to restore such a soil or make it fertile. It 

 is true that wheat maybe grown for many years in suc- 

 cession on some qualities of land without manure, but 

 experience shows that the crop gradually diminishes 

 The exhaustion may be slow, but it will surely come 



