GH. iv] Soil Losses 51 



processes that called the soil into being are still opera- 

 tive to-day; the transport of material did not come to 

 an end when the soil was brought into its present 

 position but continues, and tends to remove the soil 

 now that it is formed. The losses have gone on simul- 

 taneously with the formation of the soil and they still 

 continue. The most important source is the rain. As 

 rain falls on to the land and soaks in, it dissolves out 

 some substances and carries them away. Hence the 

 drainage-waters are always hard and often unfit for 

 drinking. The constituent removed in largest quantity 

 is calcium carbonate, no less than 8 to 10 cwts. per acre 

 of this being washed away each year at Rothamsted. 

 Other constituents, especially nitrates, are removed in 

 smaller but still important amounts. Thus in course of 

 time a soil exposed to a heavy rainfall tends to become 

 reduced to hard insoluble residues of unchanged mineral 

 fragments : finally it may become barren through loss of 

 plant food, and "sour" through absence of calcium car- 

 bonate. On the other hand a soil in a dry region of low 

 rainfall keeps all its soluble constituents intact, indeed 

 it may become so heavily charged with them as to be- 

 come barren through this very excess. Again, heavy 

 rainfall may wash the soil bodily away and leave only 

 the bare rock or a wholly impossible subsoil. This some- 

 times happens in our own country in hilly regions: a 

 serious instance occurred in 1910 in the Yorkshire Wolds 

 north of Driffield. It is not infrequent in lands of violent 

 storms, especially where man has come in and removed 

 the native vegetation that once afforded some measure 

 of protection: the eroded lands of Australia and the 

 dongas of South Africa afford instances. Wherever some 

 break in the surface of the veld allows the rain to start 



42 



