424 SCIENCE PROGRESS 



Methods of measuring Soil Reaction 



The older titrimetric methods fail to investigate completely 



soil reaction, partly because they measure only titratable 



acidity, and partly because of " adsorptive " and other buffer 



effects. Soils have frequently been arranged in the order of 



their apparent acidities by the use of litmus paper and Harris 



attempted to classify soils as " truly acid " and as what Ramann 



called " adsorptively insaturated," according to whether the 



extract alone or the moist soil itself were necessary to colour 



blue litmus paper red. Walpole and, later, Gillespie and Wise 



showed that both buffer action and time of contact were potent 



factors in the behaviour of buffered solutions towards litmus 



N 

 paper : — — — HCl in pure water has Ph =4*8, but will produce 

 10,000 



practically no effect on litmus paper apart from a certain amount 



of leaching of dye from the paper. A standard buffer phosphate 



solution, however, of Ph as great as 6 '8 reddens blue litmus 



paper almost instantaneously. The fact that moist soil itself 



reddens blue litmus paper when its aqueous extract does not 



implies no necessary difference between " truly acid " and 



" adsorptively insaturated " soils, but merely that the buffer 



action in aqueous soil extracts in the absence of the solid soil 



is not sufficient to maintain the Ph unchanged while the reaction 



of the paper itself was altering. The buffer action of soils is 



bound up with the solid phase, and the reaction of the soil 



solution is maintained constant only when in contact with the 



solid phase. It follows, therefore, that the reaction of soil 



extracts bears no relation at all to the litmus test when litmus 



paper is used and both time and buffer factors are left out of 



account. No difficulty, on the other hand, is experienced in 



testing the reaction of an aqueous soil extract when suitable 



precautions are taken and the much more brilliant and sensitive 



sulphone-phthalein dyes are used in place of litmus. 



By using a number of different indicators in combination 



with a series of standard solutions of known Ph, Sorensen in 



1909 introduced the so-called colorimetric method of measuring 



Ph- The method depends on the fact that for every indicator 



there is a particular range of Ph within which its colour changes 



but gradually. A large number of indicators are known, each 



having its own particular zone of change, which differs from that 



of most other indicators. Thus, methyl red changes its colour 



gradually from yellow through brown to red within the zone 



of Ph of 6-0 to 4*4 ; phenol phthalein changes colour between 



Ph 10 to 8 "5 ; litmus between 8 and 5 ; methyl orange, 4*0 to 



3-0. Moreover, many of these changes overlap, so that the 



tint produced on a particular indicator by a particular solution, 



