POSSIBILITY OF EXHAUSTING SOILS 285 



146. Possible exhaustion of mineral nutrients. 

 On the other hand, when we consider that the soil must 

 be depended upon to furnish food for humanity and 

 domestic animals as long as they shall continue to 

 inhabit the earth, at least so far as we now know, 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 supplement those in the soils 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 

 of the atmosphere. The last of these is inexhaustible, 

 and the exhaustion of the nitrogen supply, which a 

 few years ago was thought to be a matter of less than 

 half a century, has now ceased to cause any apprehen- 

 sion. The conservation or extension of the supply of 

 mineral nutrients is now of supreme importance. The 

 utilization of city refuse and the discovery of new 

 mineral deposits are developments well within the range 

 of possibility, but neither of these promises to afford 

 more than partial relief. The utilization of the subsoil 

 through the gradual removal by natural agencies of the 

 top soil will, without doubt, tend to constantly renew the 

 supply. The removal of top soil by wind and erosion is, 

 even on level land, a very considerable factor. The large 

 amount of sediment carried in streams immediately after a 

 rain, especially in summer, gives some idea of the extent 

 of this shifting. This affects chiefly the surface soil and 

 thereby brings the subsoil into the range of root action. 



