12 SYLVICULTURE IN THE TROPICS 



and clayey loams and also marls form heavy soils and 

 are the most resistant, sandy loams and loamy limes 

 are called mild, loamy sands and sandy marls are light, 

 ordinary sandy soils loose, and those which are liable 

 to be moved by the wind are called shifting. 



The depth of a soil has considerable importance in 

 forestry. It can be gauged according to the distance 

 to which the roots can penetrate and at the same time 

 be supplied with the necessary amount of oxygen. 

 Generally speaking, a deep soil favours the development 

 of the root-system and a corresponding development 

 of the bole or stem of a tree. Such a soil is particularly 

 needed for such trees as develop a long taproot, and a 

 check in this development may result in the tree dying 

 down from the top, or becoming, as it is called, " stag- 

 headed." On shallow soils, trees are short-boled and 

 generally slow growing. In the Tropics, where shallow 

 soils quickly dry up and get much heated, the number 

 of species capable of growing in them is restricted to 

 such as are leafless during the greater part of the year 

 or which store their own water in their stems, and to 

 some evergreens. In such soils are found different 

 species of Cactus and Euphorbia among the succulent 

 plants, and a certain number of trees belonging to the 

 natural orders of the Combretaceae (Gyroearpus and 

 Anogeissus) and Burseraceae (Bosivellia and Balsamo- 

 dendron), etc. Several species of Ficus are also 

 found growing on shallow, rocky soils, but they have 

 powerful roots which insert themselves into the fissures 

 of the rock, and are able to force themselves into the 

 moister depths. 



It is really very difficult to say what, in the Tropics, 

 really constitutes a deep or a shallow soil. French 

 foresters consider a soil very deep when it ranges over 

 90 centimetres (3 feet) from the surface, but in the 

 Tropics such a general classification cannot be adopted 

 owing to the much greater range of distribution of 

 moisture. A soil which may be deep in a damp locality 

 may be comparatively shallow in a dry or arid situation, 



