SOILS OF THE ARID AND HUMID REGIONS. 



405 



he has not determined either the true humus or its nitrogen-content, it 

 remains uncertain whether or not a sufficiency is in an available form, 

 and whether their case may not be like that of the Hawaiian soil men- 

 tioned above (chapter 19, p. 362), in which despite 10% of humus and 

 .17% of nitrogen, the land was found to be nitrogen-hungry. Again, 

 as regards the phosphoric acid, which Wohltmann considers satisfactory 

 to high, it is questionable to what extent it is rendered unavailable by 

 the very high content of ferric hydrate. We are thus left in some un- 

 certainty as to the real manurial requirements of the Samoan soils, 

 which doubtless represent very closely also those of Tutuila, the chief 

 American island of the group. 



It is probable that for crops requiring so much potash as do the 

 banana and cacao trees, potash is the first need when they cease to 

 produce well on these soils. 



Soils of Kamerun. In the soils of Kamerun, also analyzed by 

 Wohltmann, and of which two are placed alongside of those of Samoa, 

 it is at once seen that the materials from which they have been formed 

 are richer in both potash and lime than the parent rocks of the Samoan, 

 and not quite so rich in iron. They are also very rich in organic 

 matter, evidently down to the depth of a meter, as are those of Samoa. 

 It is probably due to the high humus-content that these soils, washed 

 as they have been by the second-highest rainfall in the world (about 35 

 feet annually) have not been as thoroughly leached as have been those 

 of the Brahmaputra valley. The annual rainfall of Samoa is only from 

 nine to eleven feet on the lower levels, but ranges as high as 18 feet at 

 higher elevations. 



It is noticeable that in most of these true tropical soils the 

 content of magnesia is considerably above that of lime; a fact 

 readily intelligible from the more ready solubility of lime in 

 carbonated water. It is hardly doubtful that this dispropor- 

 tion will in many cases explain a lack of thriftincss, which 

 could be effectually remedied by a simple application of lime 

 or marl, without resorting to the more costly fertilizers. 



THE SOILS OF MADAGASCAR. 



The soils of the island of Madagascar have been analyzed to 

 the number of about 500 by Miintz and Rousseaux. under the 

 auspices of the French government. 1 So large a number of 



1 Annales de la Science Agronomique, tome icr, 1901, fasicules i, 2, 3. 



