SOIL REACTION 



423 



Witte's peptone exerts its greatest effect when Ph is on the 



acid side of 5 ; but the soil colloids, from their very number and 



diversity, may exert considerable effect in all regions of Ph. 



N 

 Thus I '6^ c.c. — H2SO4 added to a mixture of 40 g. of a slightly 



acid soil and 80 c.c. of water lowered the Ph from 6-6 to 5*6; 

 the same amount of acid added to 80 c.c. pure water lowered 

 the Ph from 6-8 to 2-5. It would appear, therefore, that the 

 soil complex is well supplied with " buffers " which operate at 

 all regions of Ph- In view of these facts, it is not surprising that 

 although the lime requirements of different soils vary enormously 

 yet the variations of Ph of the same soils are confined within 



150 



—*' CCS. ^ NaOH per 100 ccs. ^ H.PO. 



o JO * 



Fig. 3. — Titration curves of 0-phosphoric acid. 



much narrower limits. The largest variations in Ph of aqueous 

 soil extracts are those given by Sharp and Hoagland — 3-7 to 

 9-7. These are relatively considerable, and include extreme 

 cases of acidity and of alkalinity. These workers found, 

 however, that the definitely fertile soils showed strikingly 

 similar reactions : slightly alkaline as indicated by Ph values 

 of 7-04 to 7-52. Such wide variations in Ph as 3*7 to 9-7 must 

 correspond to abnormally large differences in titratable acidity ; 

 on the other hand, the writer has shown that identical or nearly 

 identical values for Ph may exist in different soil samples along 

 with widely different amounts of titratable acidity as found 

 either by the ordinary lime requirements methods or by the 

 measurement of Ph- It is impossible at present to account 

 completely for the buffer actions in soils, and it can be expressed 

 only by the slopes of titration curves. 



