SOIL SCIENCE 125 



becoming evident, especially when dealing with methods of farm- 

 ing employed by Europeans in the tropics, that frequent manuring 

 is necessary to maintain fertility, and often to make the soil fertile 

 in the first place. Many native methods do succeed in maintaining 

 a certain degree of fertility, but as population increases and com- 

 mercial cultivation extends, greater demands must inevitably be 

 made on the soil. 



Soils of the arid and semi-arid parts of Africa, which make up 

 considerably more than half the total area, contain very little 

 humus, which is usually regarded as a main essential for the main- 

 tenance of fertility. In addition, some of the elements which 

 are essential for all plant and animal growth, notably calcium 

 and phosphorus, especially in the regions of high rainfall, are insuf- 

 ficient over wide areas. ^ In many areas tending to arid conditions 

 there is serious trouble from 'brak' or 'alkali', usually sodium salts 

 which are sucked up from the sub-soil under the influence of sur- 

 face evaporation, and become deposited in the superficial layers, 

 where they are often toxic to plant growth and may even form an 

 efflorescence at the surface. 



When the vegetable cover to the soil is removed, either by the 

 plough or the native hoe in order to plant crops, or by excessive 

 grazing of domestic animals, the great bane of agriculture in 

 tropical and southern Africa — soil erosion — will probably begin. 

 Even where climatic conditions or agricultural practice do not 

 lead to erosion, suitable means for keeping the ground in produc- 

 tion vary enormously from place to place according to the structure 

 and composition of the local soil. Knowledge of the original soil 

 and an appreciation of the changes involved are essential both in 

 order to increase fertility and to prevent deleterious changes which 

 may follow agricultural development, the burning of grassland and 

 other human activities. 



In treating African soils as a whole it is essential to make a dis- 

 tinction between the major geological classes. The soils of the old 

 basement complex, derived from granites, gneisses, etc., are rela- 

 tively poor, and since they spread over wide areas of the drier parts 



^ Deficiency in other minerals has been noticed in special cases, for instance the 

 unusual case of lack of sulphur, which affects some tea soils in Nyasaland (see Chapter 

 xii, p. 365). 



