CHAPTER II 

 TROPICAL SOILS 



MANY chemical changes are hastened by the application of 

 heat. This is illustrated by the rapidity with which chemical 

 changes take place in soils under the influence of constant 

 warm weather. Rocks and soils decompose with remarkable 

 rapidity in the Tropics. Even fresh lava flows in a district 

 of abundant rainfall may become sufficiently disintegrated in 

 five to ten years to furnish suitable conditions of growth for 

 a considerable variety of plants. In fact, volcanic cinder, if 

 blown out in a state of sufficiently fine fragmentation, is im- 

 mediately available as a soil. The only element of plant food 

 in which such material is deficient is nitrogen. In regions of 

 modern volcanic flows and eruptions, there are, therefore, 

 many soils of very recent origin. These soils differ greatly 

 in physical and chemical composition from the familiar soils 

 of temperate climates. The basaltic lava from volcanoes 

 may be disintegrated in place to form soils containing a mix- 

 ture of mineral elements in essentially the same proportion in 

 which they occurred in the original basalt. These soils have 

 not been altered like the old alluvial soils by ages of secondary 

 chemical changes and by the slow process of segregation of 

 certain mineral forms so characteristic of secondary rocks and 

 their resulting soils. 



Notwithstanding the recent origin of many tropical soils, 

 particularly in volcanic regions, it is obviously impossible for 

 these soils long to retain the composition of the original basaltic 

 lava. Extensive leaching takes place under the influence of 

 heavy tropical rainfall. This leaching affects not only the 



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