THE SOIL AS A CULTURE MEDIUM 



635 



obtained which can serve as an index of the buffer content of the soil. 

 The slopes of the curve vary with different soils according to their 

 buffer content. It has been suggested 47 that, although there may be 

 no correlation between the reaction of an acid soil and crop growth, 

 there is a definite correlation between the latter and the buffer content 

 of the soil. 



The concentration of carbon dioxide is of great importance in such 

 a system of measurement, especially when a relatively poorly buffered 

 soil extract is used. The actual soil solution surrounding the absorbing 

 membrane of the plant roots may be slightly acid, although the soil 

 suspension gives an electrometric measure of pH 7.0 and above. Titrati- 

 ble acidity has much wider ranges of variation due to difference in 

 buffer content. A peat or clay soil may have the same hydrogen-ion 



TABLE 67 



Influence of different nitrogenous fertilizers upon the reaction of the soit 



100 mgm. nitrogen added to 100 grams of soil 



INCUBATION 



Start . . 

 2 days 

 35 days 

 50 days 

 76 days 



concentration as a sandy soil, but the titration (or lime requirement) 

 of the first two soils will be much higher due to the greater buffer con- 

 tent. 



When nitrogen-poor organic matter is added to the soil in the form of 

 green manure or plant stubble, the first stage in the decomposition 

 results in the formation of various organic acids, particularly in the 

 absence of free calcium carbonate. 48 If aeration, temperature and 

 reaction favor a further decomposition of the organic acids thus formed, 

 there may be a change in reaction to alkalinity, due to the formation 



" Arrhenius, O. The potential acidity of soils. Soil Sci., 14: 223-232. 1922. 

 A possible correlation between the fertility of rice soils and their titration curves. 

 Ibid., 21-26. 



48 Charpentier, 1921 (p. 431). 



