eg SOIL CONDITIONS AND PLANT GROWTH 



years. His "double silicates" were never actually shown to exist in 

 the soil, but it was assumed that the reactive substances of similar 

 constitution known as zeolites, and found naturally in volcanic districts, 

 were normal soil constituents and responsible for the absorption. 

 Some justification for this view was obtained when Hall and Giming- 

 ham (121) found that the interaction between ammonium sulphate 

 and clay followed the ordinary law of mass. But the old experiments 

 of Weinhold (300) and the more recent ones of Cameron and Patten 

 (67) show that this law is not obeyed over a wider range of concen- 

 tration, and another hypothesis has therefore been developed. 



Van Bemmelen has demonstrated a close parallelism between the 

 various interchanges and absorptions shown by the soil and those 

 shown by colloids; and there is considerable evidence in other 

 directions that some of the soil constituents and especially the clay 

 possess all the properties of colloids. Now the absorption by 



V i 



colloids can generally be expressed by the equation =Kc n where 



y = the amount absorbed by a quantity m of the adsorbent ; 



c = the concentration of the dissolved substance when equilibrium 

 is attained (this can readily be expressed as (a - y) where a = the 

 initial concentration) ; 



K and n = constants depending on the nature of the solution and 

 the adsorbent. 



Wiegner (3070) has shown that the interaction between ammonium 

 salts and soil entirely accords with this reaction, and Prescott 1 finds 

 that the same is true for the adsorption of phosphates from their 

 solutions by soil. 



The adsorbed bases can readily be displaced by others, and this 

 phenomenon is utilized by Ramann in his elegant method of soil 

 analysis. 



Further light on the constitution of the soil is obtained by 

 fractional analysis. By successive extraction with acids of increasing 

 concentration van Bemmelen found (22) two distinct groups of 

 silicates in the Dutch alluvial soils, one soluble in dilute hydrochloric 



molecules of SiO 2 ., ,, , , . 



acid in which the ratio = , Al *= 3 to 5, 2 the other soluble 



molecules of A1 2 O 3 



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

 equal to 2. Other soils of volcanic origin from Java gave up larger 



1 Proc. Chem. Soc. t 1914, 30, 137. 



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 digesting the residue for a few 

 minutes at 55 with dilute alkali of sp. gr. 1*04. 



