54 SOIL CONDITIONS AND PLANT GROWTH 



sharp contrast both in composition and in chemical and physical pro- 

 perties. 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) showed 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 

 is added to the soil as manure, an insoluble nitrogen compound 2 be- 

 ing formed which remains in the soil, while the calcium sulphate 



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

 power of absorbing ammonia from ammonium sulphate. 



2 This insoluble substance does not seem to be an ordinary ammonium compound 

 since it is not completely decomposed on distillation with magnesia. [Russell (241).] 



