4 SYLVICULTURE IN THE TROPICS 



PT. I 



structure, while others require more air from the soil 

 aud less water, and that a substance which may be 

 injurious to a plant may not only not be harmful but 

 may be beneficial to others. This is, for example, the 

 case with lime, which is necessary to most plants, but a 

 mixture of more than 4 per cent of which is injurious 

 to the Sweet Chestnut. 



In examining a forest soil it will be found that the 

 top layer, which itself may or may not be covered with 

 a matting of dead and decaying leaves, is of a dark 

 colour, due to a large admixture of decomposed organic 

 matter chiefly derived from dead leaves, fruit, twigs, 

 and roots. Lower down it usually gets lighter in colour 

 as this mixture decreases in quantity, and ultimately it 

 is of the colour of the rock from which it is derived. 

 This portion of the soil is known as the sub-soil, and the 

 darker layer above as surface-soil or simply as soil. 

 The black colouring matter in the latter, which is known 

 as humus, is due to the decomposition of organic 

 matter, such as dead leaves, roots, twigs, fruit, etc. 

 This humus plays an important part not only in the 

 production of plant-food, but in modifying the physical 

 properties of the soil. Its beneficent attributes, how- 

 ever, depend on the thorough aeration of the soil, and 

 if this be interfered with, as, e.g., through an excess of 

 water in the soil or by an absence of earthworms, the 

 humus turns sour and only a limited number of plants 

 can exist on it. Among the trees which will be found 

 growing on such soils may be mentioned the Rhodo- 

 dendron. In the colder latitudes such a soil will turn 

 to peat, but in the Tropics true peat is said not to 

 form at an altitude lower than 1200 metres 1 (nearly 

 4000 ft.). 



The poverty in humus of many tropical forest soils 

 has repeatedly been commented on by different writers, 

 especially as regards the soil in evergreen forests. 

 Instead of finding, in these, a soil covered with a matting 

 of dead leaves and other vegetable remains, the lower 



1 Schimper, Plant Geography, translated by W. R. Fisher, p. 3S2. 



