SOILS OF THE ARID AND HUMID REGIONS. 



4OI 



vegetation. Nitrification must likewise, of course, be very active under 

 the continual heat and moisture, and the humus formed under these 

 circumstances is likely to be quite poor in nitrogen. On this latter 

 point, however, definite data are almost wholly wanting. 



Investigations of Tropical Soils. The most extended chemi- 

 cal investigations of properly tropical soils have been made by 

 Wohltmann in his investigations of the soils of India, German 

 Southwest and Southeast Africa, and Samoa ; l and by 

 Miintz and Rousseaux of soils collected under Government 

 auspices in Madagascar. Leather, Bamber and Mann have also 

 analyzed a large number of soils of India. But we find in 

 many of these cases a failure to specify distinctly the local 

 climatic conditions, and even the depth to which the samples 

 have been taken ; so that the investigator is obliged to examine 

 laboriously the local climates, and especially the amount and 

 distribution of rainfall, before being enabled to discuss intelli- 

 gently the data given. Even Wohltmann, in his discussion of 

 North African and Saharan soils, classes these distinctly arid 

 types among the tropical ones. 



Again, the dry seasons intervening between the tropical 

 rains, varying in length and from locality to locality, obscure 

 somewhat the relations of the soils to the climatic conditions. 

 Under the lee of mountains, even of slight altitude, we find 

 xerophytic (arid-land) vegetation, as has been noted by many 

 observers in Brazil, even near the Amazon ; in Hawaii, in 

 Jamaica, and in Madagascar. Unless, therefore, a close dis- 

 crimination is exercised by field observers, many contradictory 

 results will appear in analyses of soils of inter-tropical coun- 

 tries. This is naturally the case in India, where the topo- 

 graphic surface conformation and seasonal climatic conditions 

 are so complicated and contrasted. On the whole, the results 

 obtained in Samoa, Kamerun and Madagascar seem, of those 

 available, to be the most characteristic of true tropical condi- 

 tions. In comparing these with the soils of low plant-food 

 percentages in the temperate humid region (see chapter 19, 

 p. 352), it must be remembered that those mentioned as being 

 productive are so by virtue of great depth and relatively high 



1 J Samoa Erkundung, by F. Wohltmann, Kolonial-'Wirthsch, Komitee, Berlin, 

 1904. 



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