400 SOILS. 



availability of the nitrogen determined by the analysis. See 

 chapter 19, p. 357. 



While the continuous heat and moisture of the tropics concur 

 toward rapid rock-decomposition, it must be remembered that 

 the copious rainfall is equally conducive to an intense leaching 

 effect. Striking examples of this action occur in the Hawaiian 

 Islands, in the highly ferruginous soils resulting from the 

 decomposition of the black (pyroxenic and hornblendic) lavas 

 that are so characteristic of the volcanic effusions of that 

 region. The soils formed from these rocks are sometimes so 

 rich in ferric hydrate (iron rust) that they might well serve 

 as iron ores elsewhere. But these soils are very unretentive, 

 and though very productive at first they are soon exhausted, 

 the abundant rains having sometimes deprived them of almost 

 every vestige of lime, and of most of the potash contained in 

 the original rock. At the same time the abundant phosphoric 

 acid of the original rock has been reduced to almost total 

 unavailability by combination with ferric oxid, just as in the 

 case of the bog ore of the temperate climates ; so that phosphate 

 fertilization is urgently needed in these lands, though showing 

 high percentage of phosphoric acid. (Chapt. 19, p. 356.) 



Soils highly colored by ferric hydrate occur rather fre- 

 quently in the tropics, and have received the general name of 

 " laterite " soils. Curiously enough, the intense reddish tint 

 mostly shown in these soils, and which is emphasized in the 

 " terra roxa " of the Brazilians, and the general " red " aspect 

 of Madagascar, and of the Malabar and Bengal coasts, is by 

 no means always accompanied by markedly high percentages 

 of iron oxid ; but the latter is very finely diffused, so as to be 

 very effective in coloration. The plant-food percentages of 

 tropical soils are generally quite low, so that in the temperate 

 humid regions such lands would be adjudged to be rather poor. 

 Yet they mostly prove quite productive and lasting, even with- 

 out fertilization. 



This is doubtless to be explained by the continuous and rapid rock 

 and soil-decomposition which goes on under tropical climatic conditions, 

 already alluded to ; so as to supply enough available plant-food for the 

 demands of each season's vegetation, analogously to the proverbial 

 " nimble penny." This is supplemented also by the rapid decay and 

 eaching-out of the ash ingredients of the rapidly decaying and dying 



