C. Edmund Marshall 61 



Table I illustrates the interconnections of the three concepts of the 

 soil solution. The three ranges chosen in each case for a c cover the cases 

 most likely to be encountered. Three chloride concentrations are in- 

 cluded under a Y> and in addition the situation in the absence of salt is 

 examined, as it would be at the neutral point. Here the hydroxyl ion 

 activity is io -7 and a,. -\~ a v , is practically identical with a c . a E then rep- 

 resents hydroxide expressed from the soil. The effect of pH on the 



TABLE I 

 The Operation of the Donnan Equilibrium in Soil Systems 



Cation* a c a B a c + a B a E q — a E /a B 



Potassium io~ 2 io - ' o.oioo 3.16 X io~° 



0.0101 1.005 X I0 ~ 10.05 



0.0 1 10 3.32 X 1 o~ 2.32 



0.0200 x .41 X io~ 1 .41 



0.00 1 00 1 .00 X io" 



0. 001 01 1.005 X I0 ~ 10.05 



0.00110 3.32 X 10 3-3 2 



0.00200 1. 4 1 X 10 1. 4 1 



Calcium io~ J io - ' 0.00100 2.15X io~ 6 



0.00101 4.66 X io - 4.66 



0.00110 2.24 X io~ 2.24 



0.00200 1.26 X 10 1.26 



0.000 1 00 1 .00 X 10 



0.000101 4.66 X 1 o - 4.66 



0.000110 2.24 X 10 2.24 



0.000200 1.26 X 1 o~ 1.26 



*<<r represents the cationic activity of the soil colloids. 



a B represents the cationic activity due to soluble salts alone in presence of the soil colloids. 

 a E represents the cationic activity of the expressed solution. 



membrane hydrolysis is readily calculated from the Donnan equations. 

 For potassium each increase in pH of one unit increases the potassium 

 in the expressed liquid by x/io. For calcium the corresponding factor 

 is Vi 00. 



It can clearly be seen from Table I that large differences exist between 

 the expressed solution, the "Burdian" soil solution, and the whole soil 

 solution as here defined. As was pointed out above, these differences 

 will become even more marked in presence of carbonic acid, 



.... — /__, 





