ELEVENTH CHAPTER. 



THE SOIL AS CHEAPEST SOURCE OF 

 PLANT FOOD. 



"DESIDES the chief constituents named in pre- 

 ceding chapter, the average soil also contains 

 smaller amounts of a number of other substances, 

 especially iron in the form of oxide of iron, magne- 

 sia, soda, sulphuric acid, chlorine, and the great in- 

 dispensables — potash and phosphoric acid. The 

 percentages of any of them may be large, and of 

 some only small fractions, or even mere traces, yet 

 the aggregate amounts contained in an acre of good 

 arable soil, one foot in depth, are considerable. 

 The weight of these 43,560 cubic feet of soil, when 

 dry, will be in the neighborhood of 4,000,000 

 pounds, and in these there may be 30,000 to 40,000 

 pounds of nitrogen, 25,000 pounds of potash, 15,000 

 pounds of phosphoric acid, not to mention the sub- 

 stances of minor importance to us. These are 

 enormous quantities. The virgin soil in fertile sec- 

 tions is often chockfull of plant foods, and while 



