Atkins — Factors affecting Hydrogen Ion Concentration of Soil. 387 



carbonate, have less. buffer action, especially the sandy. That is to say, 

 percolation with an alkaline water renders them alkaline, witli an acid water 

 they become acid ; normally they are feebly acid, due to carbonic acid and 

 plant remains. Many are, however, not free from carbonates, and are as 

 alkaline as are limestone soils, though having a lower buffer action. 

 Furthermore, it appears that, owing to adsorption effects, clay soils may have 

 a very considerable buffer action. 



The hydrogen ion concentration of heeded and unheeded soil. 



ISTumerous researches emanating from Eothamsted and elsewhere have 

 shown how profound is the influence of heat in modifying the soil and its 

 extracts, and in effecting changes, both in quantity and character, in the soil 

 population. Kecent bacteriology contains many records of the influence the 

 hydrogen ion concentration exerts upon the growth of bacteria, protozoa, and 

 fungi. It is accordingly important to consider how heat may alter the soil 

 reaction. 



G-rigoriev (1916) noticed that the alkalinity of soil extracts increased 

 during evaporation. Sharp and Hoagland (1919) pointed out that the effect 

 of boiling was very slight or nil in the case of soil extracts lying between 

 pH 5-3-7-3. This has been confirmed by the writer for sandy soil in the 

 region of pH 5-4, which only altered to pH 5'7. But with alkaline soils 

 giving extracts of pTl 7'8-S-4 it was found that, on boiling, the carbonate 

 limit, slightly over pH 9'0, was reached through elimination of carbon dioxide. 

 With a soil from Bihar, originally at pH 9-0, when tested by shaking the air- 

 dried silt with boiled distilled water, pH 9-2 was reached, due probably to the 

 presence of traces of magnesium insufficient in quantity to give its maximum 

 carbonate reaction, pH 10-0. Moore (1920) has pointed out that as a rise of 

 pH 1 increases the hydroxyl ten times and diminishes the hydrion by the 

 same factor, the result is a hundred-fold alteration in the relation between 

 amphoteric colloids and their ions. This i-elation is of the utmost importance 

 in interpreting the effect of changes in hydrogen ion concentration, since 

 these are effective in proportion to their squares. The large alterations in 

 reaction occasioned by heating the extracts of alkaline soils are probably not 

 without a marked effect on the growth of bacteria, etc., subsequently in soil 

 similarly treated. As multiplication progresses, however, the production of 

 carbon dioxide restores the pH value to the neighbourhood of its initial 

 value. How closely this is regained will depend upon how far the soil has 

 been altered physically by heat treatment, which probably leaves it in a 

 rather different state as regards aeration when brought back to its initial 

 water-content. Further research on this subject seems desirable. 



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