THE CONSTITUTION OF THE SOIL 55 



represent the undecomposed minerals of the original rock : he has 

 modified the methods of mineralogical analyses and applied them to 

 soil with considerable success in the South of France. 



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 physical properties. At 

 least two groups of clay were recognised by Hall and Russell, one 

 associated with fertile soils, the other with less fertile soils. 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. Other methods have proved more fruitful, 

 and two in particular: (i) the study of the absorption by soils of 

 various ions NH 4 , K, PO 4 , and others from their aqueous solutions; 

 (2) van Bemmelen's successive extraction of soils and clays with acids 

 of increasing concentration. These we must study in some detail. 



Mention has already been made of the fact that soils precipitate 

 salts of ammonium, potassium and phosphates as well as organic com- 

 pounds form their solutions. Liebig (175) regarded this property as 

 purely physical, but Way (298) argued in a classical research that it 

 is really chemical. Starting with Thompson's observation (283) that 

 calcium sulphate goes into solution when ammonium sulphate solution 

 is shaken with soil, he showed in the first instance that the amount of 

 base dissolved out is equivalent to the amount of ammonia fixed, and 

 thus established the chemical nature of the change. His next experi- 

 ments were to discover the particular constituent of the soil with which 

 the reaction took place; he found it was neither the calcium carbonate, 

 the sand, the undecomposed rock however finely ground, nor the 

 organic matter. 1 The active constituent was in the clay, but it formed 

 only part of the clay, and moreover it lost its power on ignition. No 

 known simple silicates showed these properties, but he prepared a 

 number of " double silicates " of lime and alumina, of soda and alumina, 

 etc., that did; thus they reacted, like clay, with ammonium salts to 

 form an almost insoluble double ammonium silicate and a soluble 

 calcium salt, and also, like clay, they lost this property after ignition. 

 Although he did not establish the existence of such double silicates in 

 soil, their resemblance to the reactive constituent in the soil was so 

 close that he considered himself justified in assuming their presence. 



Further experiments by A. Voelcker (287, 288) and others have 

 shown that the same change takes place when ammonium sulphate 



1 It was subsequently shown by Konig (153) that soil organic matter has a marked 

 power of absorbing ammonia from ammonium sulphate. 



