TROPICAL SOILS 19 



however, will grow satisfactorily on highly manganiferous 

 soils. Pineapples, for example, cannot be made to yield satis- 

 factory returns on soils containing more than 2^ or 3 per cent, 

 of manganese. The presence of large quantities of manganese 

 in the soil has the peculiar effect of disturbing the mineral 

 balance, forcing the pineapple to absorb lime out of all pro- 

 portion with magnesium. The result of this unbalanced soil 

 solution is that the pineapple leaves lose their green color, be- 

 coming yellowish and finally dying, while the fruit turns pink 

 at an immature stage and always remains much more acid 

 than normal fruits. 



In a few localities, soils of a very high titanium content oc- 

 cur, the titanium sometimes amounting to 35 per cent. This 

 element, however, has no apparent effect upon the growth of 

 plants. In one restricted locality in Hawaii pineapples thrive 

 well on a soil of which iron and titanium alone constitute 85 

 per cent. Soils with a high titanium content have a bluish 

 or blapkxolor, especially when slightly moist, and will stain the 

 hands almost as effectively as a paint pigment. 



Near the seashore of all tropical islands surrounded with 

 coral reefs occur areas of soil constituted largely of coral sand. 

 The chief constituent of this sand is carbonate of lime. Coral 

 sand mixed with a small percentage of soil washed down 

 from upper levels and containing minute quantities of humus 

 is quite satisfactory for the growth of watermelons, sweet 

 potatoes, coconuts, and a number of other crops. The 

 coral sand is also extensively used for the purpose of liming 

 soils. 



One of the peculiarities of the laterite soils of the Tropics 

 is their high hygrosco£ic_ moisture content. Many of the deep, 

 rich soils of the Western States have a hygroscopic moisture 

 content of about 12 or 13 per cent. The laterite soils of the 

 > Tropics, however, may contain 24 per cent, of moisture in an 

 air-dry condition. It is quite plain, therefore, that these tropi- 

 cal soils require a much higher total moisture content for the 



