Jan. at. 1918 Relation of Carbon Dioxid to Soil Reaction 143 



represented by soils 1,4, 5, 6, 7, and 8, the neutral or slightly alkaline 

 type by soils 2, 9, 10, and 11, and the strongly alkaline reaction by soil 3. 



Considering first the case of the acid soils, we note that the increase in 

 H-ion concentration of the soil suspensions, in contact with even the 

 higher percentages of carbon dioxid, is scarcely greater than the errors 

 of observation. In soils i and 8, whose suspensions give H-ion concen- 

 trations of the magnitude io~*, there has been no perceptible change in 

 reaction due to the presence of carbon dioxid. The other acid soils, 

 which have a smaller concentration of H ion, show some slight increase 

 in acidity when in contact with the mixture of gases containing carbon 

 dioxid. This might be expected from a consideration of the relation of 

 the dissociation of carbonic acid to that of the soil acids. Those soils 

 whose acids dissociate comparably to carbonic acid are not measureably 

 affected by the partial pressures of carbon dioxid used in these experi- 

 ments, while soils containing less dissociated acids have their H-ion con- 

 centration increased to a sHght extent by the carbon dioxid. 



The H-ion concentration of suspensions of the slightly alkaline soils is 

 appreciably increased by increasing the partial pressure of the carbon 

 dioxid. The degree of increase in acidity seems to be dependent upon 

 the proportion of carbon dioxid in the gas mixture. In fact, the higher 

 percentages of carbon dioxid brought about a slightly acid reaction in 

 the suspensions of these soils. 



In case of soil 3, a so-called "alkali" soil, which had a very low H-ion 

 concentration, a large increase was caused by the introduction of carbon 

 dioxid into the hydrogen-electrode cell. 



DISCUSSION OF RESULTS 



The results recorded in this paper show that the effect of carbon dioxid 

 on the H-ion concentration of soil suspensions is not an insurmountable 

 difiSculty in obtaining the reaction of soils under various conditions by 

 means of the hydrogen electrode. The technic followed in the former 

 investigation in which the loss of carbon dioxid is minimized, although 

 not entirely avoided, evidently gives results of the same order of magni- 

 tude as would be obtained if there were no loss of carbon dioxid. The 

 adoption of such a view is warranted by the fact that the maintenance 

 of a small partial pressure of carbon dioxid above the soil suspension in 

 the hydrogen-electrode cell, only altered the H-ion concentration by 

 less than a magnitude. It should be remarked, however, that in soils 

 containing alkali carbonates, and having a high OH-ion concentration, 

 the partial pressure of the carbon dioxid exercises a very pronotmced 

 effect upon the reaction, as instanced by soil 3. For a clear exposition 

 of equilibria governing such systems the reader is referred to Johnston {4) . 



So far we have not considered the question of H-ion concentration in 

 soils under field conditions. By determining the carbon-dioxid content 

 of the soil atmosphere under field conditions and then duplicating the 



