52 An Account of the Soil [pt. i 



a little water-course, thQ washing away goes on along 

 that line. The break may be a natural depression, or it 

 may result from clearing the veld for cultivation or even 

 from keeping cattle always to one track in passing to 

 and from their drinking-places. Torrential rains soon 

 remove the soil and lead to the remarkable erosion 

 shown in Fig. 13. The remedy consists in delaying the 

 water and making it run off more slowly. 



Soil belts and climatic zones 



We have seen that right from the very commence- 

 ment of its history the soil has been moulded by the 

 climate, and it is not surprising, therefore, that parts of 

 the earth with characteristic climates should also have 

 correspondingly definite soils. Wherever there is a well- 

 marked climatic zone we may look for a well-marked 

 soil type. In classifying soils it is necessary first to 

 divide them into great groups according to the climate 

 and then to subdivide these groups according to the 

 geological origin of the material. 



These zones can be recognised in any great continental 

 area. In the great dry belt in the west of North America 

 there is a scarcity of vegetation, consequently no great 

 amount of organic matter finds its way into the soil. 

 Further, in the absence of rain recourse is had to irriga- 

 tion which leads to an accumulation of soluble salta 

 some of which are directly harmful to the plant while 

 others indirectly injure it by depriving it of such little 

 soil moisture as is present for plants can only take 

 water from weak and not from strong solutions. The 

 salts arise in part from the breaking up of certain 

 mineral particles, but in the main from pre-existing 

 inland seas or lagoons that have long since disappeared. 



