Atkins — Factors affecting Hydrogen Ion Concentration of Soil. 379 



that the error from this does not exceed pH O'Ol, namely, what is judged by 

 intensity or tint to be pHa; + O'lO is in reality pHo; + 0'09. 



Euppin has further calculated that a calcium carbonate solution in 

 equilibrium with the carbon dioxide of the air, which exerts a partial 

 pressure of 3 x 10"* atmosphere, contains five times as much of the car- 

 bonate as does a solution in water free from carbonic acid at 16° C. For 

 this the hydroxyl number is 17 x 10'', which, being converted, gives 

 pH 8'37. This, then, is the reaction to be expected in a saturated solution 

 freely exposed to the air. In the soil, however, the action of bacteria, pro- 

 tozoa, worms, fungi, and plant roots results in further production of carbon 

 dioxide, so that the equilibrium is displaced in the more acid direction. 

 Bjerrum and Gjaldbaek (1919) have determined by the electrical method 

 that a solution saturated at 18° C. with both calcium bicarbonate and car- 

 bonic acid at atmospheric pressure is at pH 5'2. The more carbon dioxide 

 and the more bicarbonate there is in the soil the more acid will be the 

 reaction up to this limit. It is improbable that these conditions are ever 

 realized in nature, nitrogen being present in preponderating amount in the 

 soil atmosphere. The state actually occurring in the soil is not, however, 

 true equilibrium, and carbon dioxide will be given off slowly into the soil 

 air, which is always much richer in this gas than is the atmosphere. The 

 importance of this has of late been emphasized by Howard and Howard 

 (1920) and by Turpin (1920). The value 0'25 per cent, has been given by 

 Keen (1921) as an average for the content of carbon dioxide in the soil air, 

 but Howard has found up to 1'9 per cent, in badly aerated soils in the rainy 

 months at Pusa. 



Thus, considering the equilibrium between carbonate, bicarbonate, 

 carbonic acid, and dissolved gaseous carbon dioxide and the disturbance 

 caused by production of the gas in the soil, it is clear that the pH value of the 

 soil solution is dynamic rather than static. This is very evident in soil extracts 

 in which those rich in organic matters rapidly become less alkaline on stand- 

 ing owing to bacterial action. To attempt to measure the pH value of 

 soils to the extremes of accuracy of which the method is capable is, there- 

 fore, in most cases waste of time from a biological standpoint, as it will vary 

 with the temperature as affecting the life of organisms, with temperature as 

 affecting the equilibrium quantity of carbon dioxide dissolved, and with the 

 moisture-content as regulating the removal of carbon dioxide from the soil, 

 as well as, indirectly, its production. 



- Wells (1915) gives the solubility of calcite in contact with the atmo- 

 sphere as varying from 81 to 70, 61, 52, 44, 38 parts per million as the 

 temperature rises from 0" to 50° C. in 10° steps. Prom these figures it 



