EIGHTH CHAPTER. 



THE MINERAL PLANT CONSTITUENTS 

 PHOSPHORUS AND POTASSIUM. 



'\17'E NOW come to a consideration of the sub- 

 stances of plant food, which with available 

 nitrogen are of more than ordinary interest to the 

 farmer. All other substances needed for plant 

 growth are so generally and abundantly provided 

 in average soils, that they need not give us any 

 serious concern. The supply of available phospho- 

 rus and potassium, like that of nitrogen, however, 

 becomes exhausted by continued cropping, and we 

 find ourselves confronted by the necessity of sup- 

 plying the deficiency, or of ceasing to produce crops. 



The element phosphorus has a strong liking for 

 oxygen, and it unites with it on the slightest provo- 

 cation. A " mere scratch, " or the least friction, is 

 suflScient to make it flare up in a sud- 



^^Acw!'**^ den outburst of fire, resulting in a vio- 

 lent union of the two elements, and 

 thus forming phosphoric acid. This is an occur- 

 rence that comes under our daily observation. 



The colored mixture at the end of a common 



