420 SOILS: PROPERTIES AND MANAGEMENT 



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 apprehension. The conser- 

 vation or extension of the suppty of mineral nutrients is 

 now of supreme importance. The utilization of city refuse 

 and the discovery of new mineral deposits are develop- 

 ments 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 topsoil will, without doubt, tend 

 to constantly renew the supply. The removal of topsoil 

 by wind and erosion is, even on level land, a very con- 

 siderable 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. 



There is little doubt that a moderate supply of plant- 

 food materials will always be available in most soils, but 

 for progressive agriculture manures must be used 



