56 



(d) SOIL REACTION. 



XXVII. E. M. Crowther. " Studies in Soil Reaction. 



III. The Determination of the Hydrogen Ion Con- 

 centration of Soil Suspensions by Means of the 

 Hydrogen Electrode/' Journal of Agricultural Science, 

 1925. Vol. XV., pp. 201-221. 



An improved hydrogen electrode apparatus is described and 

 its use illustrated by reference to a number of soils showing 

 characteristic crop failures. The buffer action of soils is repre- 

 sented by titration curves giving the equilibrium pH values 

 corresponding to additions of varied amounts of lime water. 

 Adjacent or similar soils may show considerable differences in 

 pH value with no change in their buffer action. In such cases 

 any " lime requirement " method is likely to show results which 

 are correlated with the pH values, but this cannot be the case 

 in soils of different types with different degrees of buffer action. 

 Additions of neutral salts cause considerable increases in the 

 hydrogen ion concentrations of both acid and slightly alkaline 

 soils. Sodium salts, including sodium hydroxide, always give 

 lower hydrogen ion concentrations than the corresponding potas- 

 sium or calcium salts. The titration curves of a soil in the 

 presence of different amounts of a neutral salt run parallel ; the 

 buffer action of a soil is not affected by neutral salts. Extraction 

 of a soil with water causes a considerable reduction in the 

 hydrogen ion concentration, i.e., an increase in pH value. This 

 effect may operate in wet seasons in diminishing the infertility 

 of acid soils and in increasing the stickiness of heavy soils. A 

 number of soils showed a regular decrease of 0.1 in pH value 

 for a two-fold increase in the soil-water ratio. This " dilution 

 effect " and the " salt effect " appear to result from a complex 

 equilibrium between the hydrogen ions and metallic cations, at 

 the soil surface, and form important cases of u base exchange." 

 The indicator methyl red gives erroneous results in turbid sus- 

 pensions owing to the absorption by the soil of the red form, 

 which is apparently a cation exhibiting " base exchange " with 

 the soil. 



XXVIII. E. M. Crowther. " Studies in Soil Reaction. 



IV. The Soil Reaction of Continuously Manured 

 Plots at Rothamsted and Woburn." Journal of Agri- 

 cultural Science, 1925. Vol. XV., pp. 222-231. 



The continuously manured grass plots at Rothamsted and 

 barley plots at Woburn are acid, except in one or two cases. 

 Sulphate of ammonia has caused a marked increase in acidity, 

 and nitrate of soda a slight increase. The farmyard manure 

 plot at Woburn is appreciably less acid than the unmanured. 

 Mineral manures have had little or no effect on the reaction of 

 the surface soil, but sulphate of potash has slightly increased the 

 acidity of the subsoil below the more acid plots. There is some 

 evidence that the acidity of the surface soil at Rothamsted is 

 approximating to an upper limit of pH value 3.8, where large 

 dressings ol sulphate of ammonia are applied. The change in 



