PLANT-FOOD MATERIALS IN SOILS 



109 



Table 27. — Number of Pounds of Nitrogen, Potash, Lime 

 and Phosphoric Acid Removed from One Acre of Soil by 

 Certain Crops 



While these are only a few of the cultivated crops, they 

 give some idea of the quantities of plant-food materials 

 removed from soils by ordinary cropping. The nitrogen 

 removed by red clover is partly taken from the air and conse- 

 quently the draft on the soil supply is not so great as would 

 be indicated by the figure here given. 



132. Possible exhaustion of mineral nutrients. — Com- 

 paring the figures given above with those in Table 17 

 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. On the other hand, when it is considered 

 that the soil must be depended on to furnish food for hu- 

 manity and domestic animals as long as they shall continue 

 to inhabit 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 

 becomes a matter of grave concern. 



The visible sources of supply to replace or to supplement 

 the nutrients in the soil now cultivated are, for the mineral 

 substances, the subsoil and the natural deposits of phosphates, 

 potash salts and limestone ; and for nitrogen, deposits of 

 nitrates, the by-product of coal distillation and the nitrogen 



