SOIL SCIENCE 137 



tated by soil erosion. In eastern Africa overstocking and destruc- 

 tive methods of cultivation are steadily reducing the productivity 

 of land which is required for an increasing native population. In 

 western Africa the destruction of forest has also played its part, and 

 in some areas the pressure of population on land of which the 

 fertility has been reduced to a dangerously low level provides an 

 administrative problem of some urgency. 



Nearly every report from Africa on agriculture, animal hus- 

 bandry, forestry or geology, refers to the serious nature of the ero- 

 sion problem, and most of the special enquiries bearing on agri- 

 culture have attempted to analyse the position and to suggest 

 measures for its amelioration. Among them may be mentioned 

 the report of the South African Drought Commission (1923), the 

 report of the Agricultural Commission in Kenya (1929), the 

 report of the Kenya Land Commission (1934), and the enquiry 

 in Southern Rhodesia into the economic position of the agricul- 

 tural industry (1934). These and other enquiries have borne fruit 

 by making the urgency of the problem widely known, but they 

 have been limited to certain aspects only, with the exception of 

 the South African Drought Commission, of which the report deals 

 only with a small part of the continent and will soon be out of 

 date. 



The Imperial Bureau of Soil Science produced a short report on 

 soil erosion in 1933, and a fully documented report (1938) des- 

 cribing the causes of erosion and the measures taken to control it 

 in every affected country in the world. 



Soil erosion is usually divided into sheet erosion and gully 

 (donga) erosion according as the surface soil is removed bodily 

 from wide areas of country, or ever-deepening water channels are 

 cut through the soil and underlying deposits. This distinction is 

 drawn for example, by Champion (1933) and Hobley (1933) with 

 reference to Kenya. It is perhaps more exact to divide the damag- 

 ing effects of soil erosion by water into four categories, which 

 rougly represent the stages of damage as erosion proceeds. These 

 are: i. Soil is washed from the surface of sloping land; 2. This soil 

 is deposited on flat land at the foot of the slopes, but in the con- 

 dition of infertile debris, because the original mixed ingredients 

 are sorted by water action, the coarse sands being deposited on the 



