THE SOILS OF THE CAXE-GROWIXG REGIONS 



65 



the basic rocks, which are mainly found in regions of comparatively recent 

 volcanic activity. The soils formed from the acidic rocks contain in general 

 more potash than do those of a basaltic origin, these being characterized 

 by the presence of larger quantities of iron and of lime. 



The following figures, due to Burgess- as the mean result of the anah'sis 

 of 1547 American mainland soils, and of 515 analyses of Hawaiian soils, 

 illustrate the differences in composition between soils derived from acidic 

 and those from basic rocks. The anah'ses were made by official American 

 method, i.e., digestion in hydrochloric acid of sp. gr. 1-115 for 10 hours 

 at 100° C. 



Material. 

 Total silica . . 

 Soluble silica 

 Potash 

 Soda . . 

 Lime 

 Magnesia 



American. Hawaiian. 



85-5^ 

 6-40 

 0-40 

 0'27 



0-75 

 0-68 



32-63 



17-59 



0-34 



0-35 



1-30 



American. Hawaiian. 

 0-50 

 28*02 



■Manganese oxide 0'i2 



Ferrous & ferric oxide 3* 81 



Alumina .. .. 5''^5 20-72 



Phosphoric acid o- 16 0'35 



Sulphuric acid .. 0-04 0-32 



Nitrogen .. o-i8 0-33 



Classed according to physical condition, soil physicists recognise four 

 main types of soils : gravels, sands, loams and clays. To these are to be 

 added intermediate classes as sandy loams, clay loams^ etc. The distinction 

 which is based on the size of the soil particles is entirely arbitrar\', and one 

 type passes insensibly into another. In the United States the distinction is 

 generally as indicated below — other arbitrary and allied distinctions ob- 

 taining elsewhere : — 



Gravel : — Particles greater than 0-05 inch in diameter. 



Coarse sand : — Particles with diameter lying between 0'05 and 0-02 inch diameter. 



Medium sand : — - 

 Fine sand : — ■ 

 Very fine sand : — • 

 Silt :— 

 Clav : — 



less than 



0'02 and o-oi 

 O'Oi and 0-004 

 0-004 and 0-002 

 0-002 and 0-0002 

 0-0002 inch diameter. 



A third method of classifying soils separates them into stationary and 

 transported soils. In the former class are those formed in sitii as on plat- 

 eaus and on lands of small gradient, and as such their composition reflects 

 that of the underl}dng rocks from which they are formed. Transported 

 soils are either wind or water-borne, or else have been conveyed by glacial 

 drift. When the motion is slow, as on the gentler slopes of a mountain 

 area, the term colluvial is apphed ; such a motion usuallj' takes place under 

 the influence of rainfall, ^^ilen the soil is transported by a river, and finally 

 deposited in its overflow as a silt, the term alluvial is used, the glacial drift 

 formation receiving the term diluvial. 



To these types should be added the peat and bog soils that are formed in 

 situ, and to which the term cumulose is given ; other distinctions of less 

 importance are those dift'erentiating between humid and arid soils, temperate 

 and tropical formations. 



It is at once apparent that all these distinctions overlap. A stationary 

 soil may be either basic or acidic, and an allu\nal soil will partake of the 

 nature of the material over which has flowed the river to which its forma- 

 tion is due. This may be either basic or acidic, or a combination of both 

 types, and the nature of the soil mil be influenced by the character of the 

 formation upon which the deposit is made. 



