PT. I 



6 SYLVICULTURE IN THE TROPICS 



covers large alluvial flats and depressions, and which is 

 caused by the slow deposit of particles of clay by waters 

 highly charged with them during flood time, belongs to 

 the heavier soils. When properly tilled it is a very 

 fertile soil for certain crops, and is particularly favour- 

 able to the cultivation of cotton ; but untilled and 

 unaired it is on the whole a poor forest soil, capable of 

 supporting only a limited number of trees. Neverthe- 

 less some species, notably several species of Acacia, 

 are able to form forests on cotton soil, such as, e.g., A. 

 arabica on the periodically flooded lands near rivers, 

 the A. Seyal on somewhat higher lands, the A. Suma 

 and A. Seyal var. Fistula in pans and depressions. 1 



Among other species which generally indicate a 

 clayey soil may be mentioned the Tamarind (Tamar- 

 indus indica), some of the species of Bauhinia (B. 

 racemosa and B. reticulata), several trees belonging 

 to the natural order of the Ebenaceae (Diospyros 

 mespiliformis, D. Embryopteris, Mala abyssinica, 

 etc.),^ many of the Sapotaceae (Bassia latifolia, B. 

 longifolia, Butyrospermum Parkii, etc.). 



Limey soils contain 10 per cent or more of lime, 

 usually present as a carbonate. They are fertile soils 

 of a favourable texture for admitting air and w T ater, 

 but they have the drawback of often being rather 

 shallow, and when lime is present in large quantities 

 they let the water through too easily. But in proper 

 quantities lime is favourable to the greater number of 

 plants, although there are a few species which will not 

 thrive when it is present in the soil. The forest flora 

 is generally much richer on limey soils than others. 

 Figs. 1 and 2 show the change of forest vegetation 

 which occurs in adjacent portions of forest with a 

 change of soil. They represent forest Blocks on the 

 Blue Nile, Fig. 1 showing a crop of pure Acacia Seyal 



1 Recent chemical analyses of such soils, made at the Wellcome Tropical 

 Research Laboratories at Khartoum, show that they are markedly deficient in 

 organic matter and in nitrogen. Their black colour must be due, therefore, to 

 that of the rock from which they are derived. In India, cotton soil is said to 

 be the product of disintegration of trap-rock. 



