J. Hexdrick axd W. f4. Ogg 463 



is a greater percentage (if phosphoric acid present in highly insoluble 

 forms in the finer particles of the soil ; but it is to be remembered that, 

 as has been shown above, the greater jaart of the phosphoric acid of 

 the soil was removed during the preliminary treatment with dilute 

 hydrochloric acid before the mechanical fractions were separated. 



Lime, on the other hand, shows a maximum percentage in the fine 

 sand and falls to a minimum in the clay, while potash and soda are 

 irregular, but both show their smallest percentages in the clay. 

 A similar fall in the case of lime was found by Schneider in the case of 

 the clay from the i-esiduai soil from augite-andesite already referred 

 to. This soil was derived from the Rockland Ridge, Washington. On 

 the other hand most investigators have found the highest percentages 

 of potash and lime in the finest fractions. 



Table IV shows the distribution in the diflierent mechanical fractions 

 of the constituents calculated as perceiitages of the total dry soil. The 

 interest of this table lies in the fact that it brings out clearly how large 

 a store of potential food material is present in the coarser fractions which ■ 

 constitute so large a proportion of Craibstone soil. For instance in 

 the case of lime the greatest proportion is present in the fine sand, 

 and over two-thirds of the total lime and nearly two-thirds of the 

 total potash are present in the coarse sand and fine sand taken 

 together. On the other hand the clay contains only a very minute 

 proportion of either the potash or lime of this soil. Robinson {inc. oil.) 

 considers the richness in potash of the soils of North Wales due to the 

 potash minerals in the fractions usually classed as sand and gravel. 



The connection heliveen Ihc wtture and origin of a soil and 

 the composition nj its mechanical fractions. 



While in general the percentage of silica is found to decrease and 

 the other elements to increase in the finer fractions, the relative 

 decreases and increases differ very much in dift'ereut soils. These 

 variations are connected with dift'erences in the nature of the soils and 

 in the conditions under which they were formed and at present exist. 



There have been at least three processes at work which influence the 

 composition of the fractions, and may lead to dift'erences in soils which 

 were in origin alike. 



(a) Mechanical grinding. In the grinding down of rocks in the 

 process of soil formation, whether by glacial action or by water, the soft 

 materials are most readily powdered and consequently there is a large 

 proportion of these in the finest fractions. 



