18 TROPICAL AGRICULTURE 



is quite commonly used in lawns and gardens for mixing with 

 the heavy soils to improve the aeration and the drainage. 



The so-called clay soils of certain tropical countries are 

 often clay only in mechanical analysis, not in composition. 

 These clays are not aluminium silicate but are high in their 

 content of iron and aluminium hydrates and low in silicates. 

 These soils are commonly referred to as laterite soils. The 

 term is not easily defined, but is usually taken to mean heavy 

 tropical soils formed from decomposing lava under the influ- 

 ence of heat, tropical downpours, and periods of drought. 



Laterite soils are not only extremely heavy, but pack and 

 puddle badly. The texture is injured by any manipulation 

 while the soil is too moist. Even when allowed to stand after 

 a year or two without cultivation the soils become so badly 

 packed as to become almost impervious to air and water. Dur- 

 ing periods of drought wide, deep cracks form in such soils 

 and the cracks are quickly filled again by the swelling process 

 which occurs with the return of the rainy season. As already 

 stated, however, these soils are not true clay and cannot be 

 used for the formation of brick. If bricks are made from 

 laterite soils they will readily disintegrate under the influence 

 of weather conditions. The huge lumps which are turned 

 up in plowing laterite soils gradually slack like lumps of lime 

 under the influence of moisture and sunshine. 



Mention may well be made of a few peculiar soils which oc- 

 cur in Hawaii and elsewhere in the Tropics. The most highly 

 manganiferous soils thus far studied occur in Hawaii. In the 

 mainland soils of the United States manganese may be found 

 as a mere trace and usually not to an appreciable extent. In 

 Hawaii, however, nearly all soils contain from one-quarter to 

 one-half per cent, of manganese and in certain restricted areas 

 the content of manganese rises to 10 per cent. Soils which 

 contain three per cent, of manganese or more are floury, of a 

 chocolate color, and will not pack like the ordinary laterite 

 soils, but remain always in a fine state of tilth. Not all crops, 



