THE COMPOSITION OF THE SOIL 



101 



Table XXIX. shows the difference in composition between 

 the fractions obtained by Hendrick and Ogg from the Aberdeen 

 soil, belonging to the intermediate grade, and the more com- 

 pletely washed and weathered soils studied by Hall and 

 Russell, where practically nothing but quartz had survived in 

 the coarse particles. 



So far as is known all the coarser particles are chemically 

 inert. The clay fraction, on the other hand, stands out in 

 sharp contrast both in composition and in chemical and physi- 

 cal properties. 1 At least two groups of clay were recognised 

 by Hall and Russell, one associated with fertile soils, the other 

 with less fertile soils. The high proportion of iron and alumina 

 in the latter case is not causally connected with infertility as 

 the clay from fertile soils in North Wales contained even more 

 (Robinson (240)). The analytical figures throw very little light 

 on the constitution of the clay beyond showing that it is not a 

 simple silicate expressible by a definite chemical formula. 



By successive extraction with acids o'f increasing concen- 

 tration van Bemmelen found (22) two distinct groups of silicates 

 in the Dutch alluvial soils, one soluble in dilute hydrochloric 

 molecules of SiCX 



acid in which the ratio 



= 3 to 5, 2 the other 



molecules of A1 2 O & 

 soluble only in hot, strong sulphuric acid in which the ratio is 



1 For some comparable German analyses of clay see E. Blanck, Landw. 

 Versuchs-Stat., 1918, 91, 85-92. 



2 The higher numbers were obtained from sandy clays and the lower from 

 heavy clays. As the silica was insoluble in the acid it was extracted by digest- 

 ing the residue for a few minutes at 55 with dilute alkali of sp. gr. 1-04. 



