ii6 SOIL CONDITIONS AND PLANT GROWTH 



In consequence of the buffer action already mentioned 

 these values are not liable to disturbance by small additions 

 of substances to the soil. 



A large number of determinations of the P H or - log (H') 

 values for American soils have been made by Gillespie (1050) 

 and G. Sharp and D. R. Hoagland (260), while for British 

 soils a beginning has been made in the Rothamsted laboratories. 

 Some of the values are as follows : 



The acidity of root sap is of the order of 5 '5 -6 '8 (p. 81). 



The relatively small range of variation in comparison with 

 the large variation in amount of titratable acidity is explained 

 by the buffer action described above. 



In addition to and probably distinct from this natural 

 acidity it is possible to induce acidity in soils deficient in 

 calcium carbonate by the long-continued application of am- 

 monium sulphate. This was first observed by Wheeler in 

 Rhode Island (303) and it is demonstrated in a remarkable 

 manner at the Royal Agricultural Society's Experiment Station 

 at Woburn. Apparently this is not the same as " acidity " 

 liberated by solutions of neutral salts in acids, because am- 

 monium sulphate appears to act specifically, no other fertiliser 

 behaving in this manner at Woburn. 1 



Hall supposes, with considerable probability, that the 

 ammonia is taken up by the plant, leaving the sulphuric acid 

 in the soil, and this view seems justified by the fact that 

 nitrate of soda behaves altogether differently, leaving an alkaline 

 residue in the soil. 



1 There are a few cases on record where potassium salts are said to have 

 reduced yields, and here it is possible that acid substances have been liberated 

 by the dissolved salt. 



