ABSORPTION OF NUTRITIVE SALTS 



419 



the quantities of nitrogen, potassium, phosphorus, and 

 lime removed from an acre of soil by some of the common 

 crops. The entire harvested crop is included : 



340. Quantities of plant-food materials contained in 

 soils. Comparing the figures given above with those 

 showing the percentages of the fertilizing constituents in 

 certain soils, it is evident that there is a supply in most 

 arable soils that will afford nutriment for average crops 

 for a very long period of time. (See pars. 46, 48, 52, 53.) 



341. Possible exhaustion of mineral nutrients. On 

 the other hand, when it is considered that the soil must 

 be depended upon to furnish food for humanity and 

 domestic animals as long as they shall continue to in- 

 habit the earth, at least so far as is now known, the very 

 apparent possibility of exhausting, even in a period of 

 several hundred years, the supply of plant nutrients be- 

 comes a matter of grave concern. The visible sources of 

 supply, to replace or supplement those in the soils now 

 cultivated, are, for the mineral substances, the subsoil 

 and the natural deposits of phosphates, potash salts, and 



