TWELFTH CHAPTER. 



THE ESSENTIAL PLANT FOODS AND THEIR 

 TRADE VALUES. 



TN OUR lists of elementary bodies and simple 

 compounds we find only three about which we 

 have occasion to worry to any considerable extent. 

 These are nitrogen, potash and phosphoric acid. 

 All the rest are generally provided in greater abun- 

 dance in any soil than needed for the fullest 

 development of the plants. Any application of 

 manure, or fertilizers of any kind, is made for no 

 other purpose than to provide our growing crops 

 with one, or two, or all three of these substances, 

 and even in case of application of lime, plaster, salt, 

 etc., which in themselves contain no plant foods, our 

 object is usually in the direction of freeing one or 

 the other of these nutrients from the grasp of other 

 substances with which they have formed insoluble 

 compounds in the soil. 



Speaking of nitrogen, it may be well to call par- 

 ticular attention to its relation with ammonia. 

 Writers, dealers in fertilizers, and growers, are in 



