REMOVAL OF NUTEIBNTS FROM THE SOIL 309 



nificant data are obtained. The losses of nutrients by drain- 

 age and rotation cropping are from the figures already quoted 

 regarding the Cornell lysimeter soils. 



Table LXIX 



SHOWING THE NUMBER OF YEARS A SOIL TO THE DEPTH OF FOUR 



FEET WOULD SUPPLY NUTRIENTS FOR CROP GROWTH, 



PROVIDING THAT ALL OF THESE CONSTITUENTS 



WERE UNIFORMLY AVAILABLE 



Constituents 



Pounds to 



THE Depth of 



Four Feet 



Pounds 



Removed 

 Annually by 

 Cropping and 



Drainage 



Years 



SO. 



12,000 

 85,000 

 16,000 

 15,000 

 250,000 



370.2 

 43,5 

 46.8^ 



174.1 



142 



CaO 



P,0. 



229 

 367 



N 



303 



K,0 



1,435 



While the subsoil supplies large amounts of plant nutrients, 

 it must be remembered that only a small proportion of the 

 soil constituents, especially the phosphoric acid and potash, 

 ever become available either in surface or subsoil. Moreover, 

 crop yields decrease as the nutrients, even those most readily 

 available, are reduced. The above figures for duration of 

 crop growth are, as a consequence, merely conventional but 

 they indicate the probability of even a very fertile soil be- 

 coming quickly exhausted. 



Moreover, when it is considered that the soil must be de- 

 pended on to furnish food for humanity and domestic animals 

 as long as they shall continue to inhabit the earth, the supply 

 of plant nutrients becomes a matter of grave concern. 



The visible sources of supply, to replace or supplement 



* Sixty-five pounds of SOg are added an acre eaeh year in rain- 

 water while 31 pounds of N are added yearly to the acre in rain and 

 through the free-fixing activity of organisms (pars. 222, 236 and 238). 



