130 Phenomenon of AhsoijMon in its Relation to Soils 



of the base of the litmus by the soil while the free acid is left on the 

 paper. Many soils which give neutral solutions to water give acid 

 extracts to neutral salt solutions and Veitch^ shows that in some cases 

 of sour soils, iron and alumina are present in the extract. Salts of these 

 bases are easily hydrolysed, giving acid solutions; the acidity of the 

 soil therefore is indicated by the circumstances that the iron and 

 aluminium occur in a form easily turned out by another base. G. Dai- 

 kuhara^ has shown that in many cases the aluminium present is 

 equivalent to the acidity. The acidity of the extract also varies with 

 the concentration of the neutral salt solution used, there being a normal 

 absorption equilibrium. 



With most English soils showing acidity or sourness the defect 

 can usually be remedied by the addition of lime or chalk to the soil; 

 H. B. Hutchinson and K. MacLennan^ have shown that such soils have 

 a high absorptive power for calcium bicarbonate solution and they 

 use the expression "lime requirement" to indicate the amount of bicar- 

 bonate necessary to saturate the soil. 



It will be seen that there is a general tendency to associate all soil 

 absorptions with the general phenomenon of adsorption, and indeed 

 all the characteristics of adsorption are reproduced in most soil absorp- 

 tions. Whitney and Cameron lay great emphasis on adsorption as the 

 factor determining the composition of the soil solution although it may 

 be doubted whether absorption suffices to keep the concentration of the 

 soil solution within certain limits. 



The actual portions of the soil which take part in the various types 

 of absorption have yet to be determined — probably all of them play 

 a certain part — even sand shows a measurable adsorption for dyestuffs. 

 The absorption of bases by exchange — the Basenaustausch of the 

 Germans — is more complicated, but the phenomenon is shown by most 

 diverse absorbents. In the soil there probably exists a complex of 

 silica, alumina, possibly humus and iron oxide, which behaves like 

 Van Bemmelen's gels or Way's complex silicates. Certainly combina- 

 tions exist in soils which are easily decomposed in the cold by dilute 

 acids with the appearance of aluminium and silica in the solution. 



In another paper the influence of absorption on analytical methods 



is demonstrated and probably we are working towards a more precise 



definition of available plant foods based on our knowledge of how the 



soil absorbs them in the first instance. 



1 J. Amcr. Chem. Soc. 1904, 26, B37. 



- Bull. Imp. Cent. A(j. Exjd. Slat. Japan, 2, 1. ^ This Journal, 1915, 7, 75. 



{Received Ajyril 5th, 1910.) 



