140 Journal of Agricultural Research voi. xii, N0.3 



certain inherent difficulties. The reaction is so prolonged either by the 

 slow rate of solution of the soil acids or their slow diffusion through the 

 soil particles that the point of neutrality may not be easy to establish 

 and maintain permanently. Thus an apparent state of equilibrium at 

 the neutral point may be attained with a subsequent slow return to an 

 acid condition, owing to the solution and diffusion of the soil acids. 

 But once the total soil acids have been neutralized, a further return to 

 an acid condition can come about only through leaching processes or, 

 possibly, in a few cases through decomposition of organic matter. 



As pointed out, some soils whose solutions are neutral or alkaline 

 remove considerable quantities of calcium hydroxid from the solution 

 without materially increasing the OH-ion concentration of the soil 

 suspension. In certain cases this reaction might be erroneously attrib- 

 uted to the soil acids. Although the term "lime requirement" is in 

 common usage in agricultural literature, it has been variously interpreted 

 by different investigators. At present the term is devoid of scientific 

 significance. In distinction thereto soil reaction whether acid, neutral, 

 or alkaline, is capable of precise definition and determination. 



EFFECT OF CARBON DIOXID ON SOIL REACTION 



In considering the effect of carbon dioxid on soil reaction Maclntire 

 (8) states that many acid soils when extracted with water saturated 

 with carbon dioxid yield alkaline extracts. He also makes the following 

 conclusion : 



Since we admit that the soil solution is the medium through which a plant absorbs 

 its mineral supply, we are compelled to conclude that a plant's sotu-ce of nutrition 

 is almost always alkaline, but of varying degrees of alkalinity. 



From the nature of the chemical equilibria involved {2, 9, 4), we have 

 been unable to reach the conclusion that a solution existing in contact 

 with an acid soil can ever become alkaline owing to any change in the 

 partial pressure of carbon dioxid. An increase in carbon-dioxid tension 

 would either be without effect upon the H-ion concentration, or else 

 would increase it, depending upon the relative dissociation constants of 

 carbonic acid and the soil acids. It is conceivable that certain acid soils 

 when extracted with water saturated with carbon dioxid might yield 

 filtrates which would give an alkaline reaction after their carbon-dioxid 

 content had come into equilibrium with the partial pressure of atmos- 

 pheric carbon dioxid, but the equilibria governing the reaction of the soil 

 solution in contact with the soil are by no means identical with those 

 regulating the reaction of the filtrate obtained from such a soil. 



In order to obtain direct evidence on the influence of carbon dioxid on 

 soil reaction, the experiments reported in this paper were undertaken. 

 The method of procedure was similar to that previously described by the 

 authors, with the addition of an arrangement for controlling the partial 

 pressure of carbon dioxid in the atmosphere above the soil. By adopting 



