310 NATURE AND PROPERTIES OF SOILS 



those in the soils now cultivated, are, for the mineral sub- 

 stances, the subsoil and the natural deposits of phosphates, 

 potash salts, and limestone; and for nitrogen, deposits of 

 nitrate, the by-products 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 conservation or extension of the supply of mineral 

 nutrients is now of extreme 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 nat- 

 ural agencies of the topsoil will, without doubt, tend constantly 

 to renew the supply. The removal of topsoil by wind and 

 water 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 ex- 

 tent 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 nu- 

 trients will always be available in most soils, but for progres- 

 sive agriculture the use of green-manures, legumes and farm 

 manures must be supplemented by judicious and economical 

 application of lime and certain fertilizer constituents. 



