CHAPTER V 

 SOIL SCIENCE 



INTRODUCIION 



THE geological structure of the underlying rocks, together with 

 the long-continued action of climate and of vegetation, deter- 

 mine the type of soil in any area. Animals, agriculture, and indeed 

 the whole life of primitive peoples depend on this soil-vegetation 

 unit. Therefore soil science, which is often regarded as a special 

 branch of agriculture, is given prominence in this volume between 

 geology and meteorology on the one hand and the biological sub- 

 jects on the other. 



Since agriculture began, the importance of differences of soil 

 from place to place has been recognized, and the most primitive 

 native agriculturalist realizes that vegetation, or the lack of it, 

 affects soil fertility. Hence there has grown up the common prac- 

 tice known as shifting cultivation, which consists of allowing land 

 to lie fallow and revert to grass, bush, or forest between short 

 periods of cultivation.^ This process can be envisaged as a primi- 

 tive type of crop rotation in which forest is an intermediate crop. 

 In general the members of cultivating tribes are extraordinarily 

 good judges of land and will nearly always pick the area which 

 has the most productive soil, but the difficulty of clearing new land 

 and the type of timber required for domestic purposes also in- 

 fluences their choice. The underlying principles, according to 

 which the type of soil controls vegetation and changes in vegeta- 

 tion affect soil fertility, have been studied scientifically only in 

 recent years, and therefore soil science (or pedology, as it is now 

 often called) is still in a state of flux compared with other more 

 established branches of agricultural science. 



^ Shifting cultivation is considered in more detail in Chapter xiii, p. 376. 



