4 AGRICULTURE IN THE TROPICS [PT. I 



Soils. Speaking generally, the soils of the tropics are 

 very poor as compared with those of the temperate zone. 

 Instead of the comparatively dark colour and damp look of 

 the latter, which is partly due to the larger content of humus, 

 or decaying organic matter, they show a light colour and rather 

 dry appearance in ordinary fine weather, being very poor in 

 humus. Decay takes place so rapidly and so completely that 

 there is but little accumulation of its products. 



The soils that occur are of every conceivable kind, depending 

 mainly upon the subjacent rocks. The richest are in general 

 the volcanic soils such as are found in Java, the West Indies, 

 and elsewhere, but there are also very good deposited or alluvial 

 soils in Ceylon, India, the Malay States, and in other countries. 

 And it is worthy of notice, as tending to show that superior 

 richness, provided that the two soils have both all the elements 

 of plant food in fair amount, often makes but little difference, 

 that the growth of most cultivated crops in Java, such as tea, 

 cacao, rubber, etc., is but little superior to that in the poor soils 

 of Ceylon, and sometimes is not so good 1 . 



Probably as the result of the lack of humus, the tropical 

 soils seem on the whole to be rather less water-retaining than 

 those of more northern countries, and the plants growing in 

 them (in the open) tend to flag rather more quickly for lack of 

 water, though it is true that the sun is so much hotter that 

 this may probably be sufficient to account for the fact. 



1 This, formerly an unexplained fact, becomes clearer in the light of recent 

 physiological work. Cf. Blackman, Optima and Limiting Factors, Annals of 

 Botany, xix, 1905, p. 281 ; Smith, Application of the theory of Limiting Factors 

 to Measurements of Growth, Annals E. B. G. Peradeniya, in, 1906, p. 303. 



