264 NATURE AND PROPERTIES OF SOILS 



tion. The tenacity with which soils absorb and hold phos- 

 phoric acid is probably due to the change that the soluble 

 form undergoes almost immediately in the soil, 1 producing 

 the sparingly soluble tri-calcium phosphate (Ca 3 (P0 4 ) 2 ) or 

 the practically insoluble iron and aluminum phosphates 

 (FeP0 4 and A1POJ. 



While it is generally considered that most of the material 

 absorbed by soil, whether the action is chemical or physical, 

 is concentrated at the surfaces of the solid material, there is 

 some evidence that part of it penetrates, forming a solid solu- 

 tion. For example, the longer a gas is held at high pressure 

 within an absorbing material, the less will be released when 

 the pressure is lowered. Again, while most absorption is 

 almost instantaneous, the final equilibrium is very slow. Such 

 phenomena have given rise to a theory of molecular invasion. 



In the soil it is impossible to know whether the absorption 

 of any material has been purely physical, purely chemical, or 

 due to both actions. In all probability both types of fixation 

 occur. When a potassium compound is added to a soil, the 

 potassium is taken up very readily. The fixation at first is 

 probably physical. This type of absorption generates chem- 

 ical reactions catalytically and the remainder, and possibly 

 the greater proportion of the fixation, is probably chemical 

 in nature. 



139. Causes of absorption. — Way 2 was the first to ad- 

 vance any definite explanation of absorption. After study- 

 ing the absorptive capacity of double silicates of sodium and 

 aluminum, he decided that the phenomenon was purely chem- 



1 CaH 4 (P0 4 ) 2 + 2CaH 2 (C0 8 ) 2 = Ca,(P0 4 ) 2 + 4H 2 + 40O 2 

 Soluble Insoluble 



2 Way, J. T., On the Tower of Soils to Absorb Manure; Jour. Koy. 

 Agr. Soc, England, Vol. 11, pp. 313-379, 1850. Also, On the Power of 

 Soils to Absorb Manure; Jour. Boy. Agr. Soc, England, Vol. 13, pp. 

 123-143, 1852. Also, On the Influence of Lime on the "Absorptive 

 Properties" of Soils; Jour. Eoy. Agr. Soc, England, Vol. 15, pp. 491- 

 515, 1854. 



