TROPICAL SOILS 17 



original constituents of the disintegrated lava, but also the 

 fertilizer materials which are applied to such soils. Lime and 

 nitrate of soda are readily leached out of volcanic soils, while 

 ammonium sulphate and phosphates are fixed in the soils to 

 a rather surprising extent. The humus content of tropical soils 

 is ordinarily high. This is djje to the great mass of vegetation 

 produced under tropical conditions. If, however, there are 

 long intervals between the rainy seasons, the humus in soils 

 is rapidly decomposed under the influence of heat and aeration. 

 ^ Practically all tropical soils contain a relatively high per- 

 centage of iron. In Hawaii, the iron content of soils is lo to 

 45 per cent, (usually about 20 per cent.) ; in Samoa, 15 to 

 20 per cent. ; in Kamerun, 7 to 14 per cent. ; in Madagascar, 

 about 10 per cent. ; and in India, 2 to 48 per cent. This iron 

 commonly exists in the condition of three oxids, the ferrous, 

 ferric, and magnetic. Fortunately for the farmer the ferrous 

 iron is usually very insoluble except when the soil becomes 

 puddled so that suitable aeration can not take place. Granules 

 of magnetic iron oxid are of much more frequent occurrence 

 in volcanic soils than in the old soils of temperate climates. 

 In Hawaii, for example, magnetic iron is present to an appreci- 

 able extent in all soils, as may be seen by passing a magnet 

 over a sample of pulverized dry soil. These magnetic iron 

 granules are black, but soon assume the red color of ferric oxid 

 as the result of further oxidation. In certain localities there 

 are immense quantities of volcanic cinder or black sand which 

 have resulted from volcanic explosions. Several crops make 

 a satisfactory growth upon pure deposits of this volcanic cin- 

 der. In some localities in Hawaii the cinder contains a much 

 higher content of potash than the ordinary lava resulting from 

 a flow. While the lava obtained from ordinary flows contains 

 slightly less than one per cent, of potash the cinder may show 

 from two to five per cent, of potash. This potash is, however, 

 not immediately available, but becomes soluble slowly through 

 the gradual disintegration of the cinder. The volcanic cinder 



