amm 



10 SYLVICULTURE IN THE TROPICS pt. i 



Such soils are not suitable for plant growth, as they 

 combine some of the defects of sandy soils with those 

 of heavy clays. 



Apart from these three classes of soil there are 

 numerous mixtures which may contain, in varying 

 proportions, clay, lime, and sand. 



The loams, for example, are mixtures of clay and 

 sand, and, according to whether clay or sand pre- 

 dominates, they are called clayey loams or sandy loams. 

 They are among the most favourable for forest growth, 

 for the sand corrects the impermeability of the clay, 

 while the clay in the soil retains moisture which the 

 sand is unable to keep. 



For the Tropics no description of soil would be 

 complete without a mention of one which is largely 

 represented both in Asia and Africa, and which is 

 known as laterite. It consists usually of a red clayey 

 loam or loamy sand containing ferric oxide or ferric 

 hydrate in varying proportions. It is considered a 

 poor soil, but yet bears forests where the rainfall is 

 sufficient. 



Marls are mixtures of clay and lime and are of 

 great value in agriculture. For this reason, where they 

 are to be found, forests usually give way to field crops, 

 but they are as excellent for tree growth as for 

 agriculture. 



Reference has been made above to the physical 

 properties of the soil. I shall therefore recapitulate 

 them. They are : (a) the faculty of absorbing and 

 retaining water ; (b) tenacity or cohesiveness ; and 

 (c) depth. 



The faculty of absorbing and retaining moisture 

 has a direct effect on the amount of air which can 

 penetrate into the soil ; a soil which gets gorged with 

 water excludes the air, and as air and water are of 

 equal importance to plant life, such a soil is even less 

 favourable to plants than one which gives up its water 

 too easily, for no soil becomes so dry that it does not 

 retain some traces of moisture. Even air-dried samples 



