100 AMERICAN MANURES. 



affords a broad field for speculation and research, 

 and, when fully understood, will be a guide to 

 the fixing of the relative values of the different 

 elements essential to the growth of plants. 



The reader who has carefully perused our 

 work thus far, should have acquired a general 

 knowledge of the different substances entering 

 into the growth of plants. He should know 

 why some soils will produce one crop and not 

 another, and why two crops of the same kind 

 cannot profitably be raised in succession, on the 

 same land. Also, that some crops take from the 

 land more of one substance than of another ; and 

 that wheat exhausts the soil of its most valuable 

 constituents much sooner than corn, rye or oats. 

 He should understand the true philosophy of the 

 rotation of crops ; also, why the straw of wheat 

 may flourish and not the ear, as the straw con- 

 tains comparatively little of the same ingredients 

 required for the grain ; and why good crops of 

 wheat fall to the ground for want of proper 

 strength in the straw to support the ears, as the 

 straw requires a large amount of silica, with 

 little lime, magnesia and phosphoric acid, and 

 the grain a large amount of the latter substances. 

 In a word, he should understand why some soils 

 will grow good straw with small ears, and other 

 soils large ears with little straw. 



The amount of inorganic food required for 



