THE CONSTITUTION OF THE SOIL 77 



bute this solution. But the view that it is unalterable in composition 

 has led to some highly controversial deductions. In particular, soluble 

 fertilisers like potassium salts are not supposed to increase the amount 

 of food available to the plant, but to owe their beneficial effects to in- 

 direct actions in the soil, such as the precipitation of toxic substances, 

 facilitation of movements of soil water, etc. 



The fundamental differences between the two views are: (i) the 

 nature of the colloidal substances of the soil ; these are supposed by 

 van Bemmelen and his school to be decomposition products of weathered 

 silicates, and by Whitney to be particles of any composition provided the 

 size is sufficiently small ; (2) the constitution of the soil solution, van 

 Bemmelen supposing it to be in equilibrium with a solid solution or 

 colloidal complex, and therefore to depend as to its concentration on 

 the masses of its constituents present in the complex, while Whitney 

 supposes it to be in equilibrium with definite silicates and to be con- 

 stant in concentration. 



As the writer has shown elsewhere, 1 the evidence is rather against 

 Whitney's view. The discussion that has gone on has, however, 

 proved very fruitful in correcting the extreme chemical views once 

 held and bringing out the fact first indicated by Schiibler (253), but 

 emphasised by Whitney, that the physical properties of the soil particles 

 play a highly important part in determining fertility. 



1 Science Progress, July, 1911, p. 135. 



