Jan. 21. 1918 Relation of Carbon Dioxid to Soil Reaction 145 



One of Bouyoucos's (i) conclusions from his valuable and ingenious 

 application of the freezing-point method to soil investigations is as fol- 

 lows: 



Since no mineral soil out of a great number tested gave an acid curve but only an 

 absorption curve, and inasmucb as the free acid, and acid salt produced in these soils 

 when they were treated with neutral salts, or acid and acid salts, were carried away by 

 washing and the soils then gave an absorption ctu^ve, the conclusion seems to be that 

 the presence of soluble acids, or acid salts, in the mineral soils under favorable natural 

 conditions is only temporary, if ever present, and never permanent. The acidity 

 or lime requirement of soils, therefore, seems to be due mainly to the insoluble acids 

 of the soil, the silicic acid, silica, acid alumino-silicates, and perhaps to the insoluble 

 organic matter. There appears to be then practically no active acidity in the mineral 

 soils, but only negative. Exceptions to these general statements are probably very 

 few. 



Contrary to the above conclusion, the data presented in this and other 

 papers {3, 11), show that the solution in equilibrium with the soil par- 

 ticles of certain soils contains H ions in excess of OH ions. Such soils 

 are therefore necessarily acid and they include various types, many of 

 which would be called mineral soils. Furthermore, it is well to bear in 

 mind that the hydrogen electrode is capable of measuring specifically the 

 H-ion concentration, while the freezing-point method is unsuited to this 

 purpose. This is especially true in dealing with such heterogeneous 

 systems as those of the soil mass. These statements are not to be con- 

 strued as denying the possible value of the freezing-point method in 

 estimating the total "lime requirement." 



The soil acids are frequently referred to as insoluble, but such insolu- 

 bility is, as a matter of fact, only relative, for complete insolubility is 

 practically unknown and the soil acids must therefore have a definite 

 solubility although it may be slight. The important consideration is that 

 the solution of an acid soil must be continuously acid in just the same 

 way that a solution in contact with silicic acid or other slightly soluble 

 acids is always acid. It is quite true that when the soil is treated with a 

 base in order to bring about a condition of alkalinity, by far the greater 

 part of the base is used in combining with acids which at any given 

 moment were not in the solution. But this is in no way opposed to the 

 conclusion that the solution in contact with an acid soil is an acid solution 

 and would accordingly offer an acid medium for plant growth. The 

 effect of such a medium would be related to its H-ion concentration and 

 not to the total quantity of base required to neutralize all the soil acids 

 present. In other words, the reaction of a soil is concerned with the 

 dissolved fraction, which is in equilibrium with the undissolved soil mass. 

 If any added substance — for example, calcium carbonate — disturbs this 

 equilibrium, then it is clear that the undissolved portion of the soil will 

 enter into the reaction in accordance with the laws of mass action. 



