124 SCIENCE IN AFRICA 



Until the war period, the geological nature of the country and 

 the composition of underlying rocks were considered of prime im- 

 portance in the formation of soils. After the war, when the work 

 of the Russian school of pedologists became recognized in western 

 Europe, climate was stressed as a factor of even more importance, 

 and some authorities went so far as to claim that almost any kinds 

 of rock, subjected to identical climate, would be reduced in time 

 to the same type of soil. The last few years have seen a swing back 

 from the 'dominance of climate' view, and climate is now gener- 

 ally ranked as a contributory factor in soil formation, together 

 with rock structure, land morphology, vegetation and intervention 

 by animals and man. In the tropics it is fully realized that 

 the effect of climate on soil is much more profound than was 

 thought twenty-five years ago. Many soil properties are now 

 recognized as being of primary importance : among them are the 

 varying soil texture, the distribution and nature of the humus, the 

 constitution of the clay-substance, and the degree of acidity or 

 alkalinity. The vertical movement of soil moisture and its seasonal 

 fluctuation are now known to be of fundamental importance, and 

 to depend on the interaction of numerous factors. Progress in the 

 domain of colloid physics and chemistry has shown that the nature 

 of the water-holding forces in the soil is more important than the 

 total water-content. All of these factors differ not only from one 

 soil to another but at different depths in the same soil, and are 

 intimately bound up with its nature and suitability for different 

 crops. 



There is a prevalent idea among laymen that tropical conditions, 

 except in desert regions of very low rainfall, imply exuberant vege- 

 tation and rich soil which lends itself admirably to agriculture by 

 the simple process of removing the indigenous vegetation and in- 

 troducing crop plants. How far this idea is from the truth is shown 

 by the mass of intricate research which has become necessary with 

 almost every crop. As knowledge concerning tropical soils pro- 

 gresses, it becomes more and more evident that to judge them from 

 a European standpoint is entirely misleading. For instance, soils 

 which have so high a clay content as to be totally unfit for cultiva- 

 tion in a temperate climate, are sometimes found to break down 

 and become friable under tropical conditions. Against this it is 



