CHAPTER XX 

 THE INCREASE OF SOIL-NITROGEN 



SCIENTIFIC agriculture has been called on within 

 the last one hundred years to solve an almost endless 

 number of problems. Some of these have been solved 

 without difficulty, others only after they have taxed 

 the efforts of many men of science. Many are yet to 

 be solved. Of all the problems that have been at last 

 made clear, none has created so much discussion or 

 stimulated so much research as that concerning the 

 source of nitrogen to plants. 



Since it became known, toward the end of the eigh- 

 teenth century, that we are moving about at the bottom 

 of a great ocean composed mainly of two gases, nitro- 

 gen and oxygen, the question as to the part played by 

 the former in the growth of plants has been before scien- 

 tists. Chemists realized in the early years of the last 

 century that the rocks of the earth's crust do not ordi- 

 narily contain compounds of nitrogen. Not many years 

 later, it became known that very productive soils contain 

 five to ten thousand pounds of nitrogen per acre to a 

 depth of one foot, that many soils contain much greater 

 amounts, and that all this vast quantity of combined 

 nitrogen in the earth's surface has been derived in some 

 way from the gaseous nitrogen of the air. 



(190) 



