•376 Scientific Proceeding s. Royal Dublin Society. 



drops of indicators in pairs of test tubes. In one set the full acid tint is 

 developed ; in the other the full alkaline tint. On looking through the 

 pairs of tubes intermediate shades are seen. The details are given by 

 Barnett and Chapman (1918), Gillespie (1920), Van Alstine (1920), and 

 Medalia (1920). 



(g) Methods of determining soil reaction. — Numerous measurements of 

 the hydrogen ion concentration of soil extracts have been carried out 

 by the potentiometer method, notably by Fischer (1914), Sharp and Hoagland 

 (1916), Gillespie (1916), Kappen (1916), Plummer (1918), and many 

 recent workers. To establish the applicability of the colorimetric method 

 to soil investigations, Gillespie (1916) carried out a comparison, using the 

 indicators and standard buffer solutions introduced by Clark and Lubs. 

 He found such a reasonably good agreement as to warrant the general use 

 of the colorimetric method. The agreement holds to within pH 0-2-0-4, 

 but the divergence is usually less. Though the electrical method is regarded 

 as the standard, it is for some solutions open to serious objections owing 

 to chemical changes, such as the reduction of nitrates taking place on the 

 platinum black. Difficulties are encountered also in ensuring that the 

 dissolved gases are not swept out by the hydrogen stream ; and though 

 these have been surmounted, both for rapidity and delicacy to a degree 

 well within that required for most biological work, the colorimetric method 

 is to be preferred when based upon carefully prepared standards, which 

 should be checked by the electrical method, if possible, for specially 

 accurate work. The determination of the reaction of the soil has found 

 very numerous applications in agriculture and the related sciences ; and 

 since, so far as the author is aware, they have not as yet been utilized in 

 Irish agriculture, a short account will be given of a few of these. 



Agricultural application of studies on soil reaction. 



The methods have been used, for example, in determining the lime 

 requirements of acid soils by Joffe (1920), Blair and Prince (1920), and by 

 Fisher (1921). Now the soil acidity may in itself be injurious if sufticieirtly 

 great, or it may simply denote deficiency in alkaline calcium salts, to a 

 shortage of which the evil resuks may in i-eality be due, or acidity may lead to 

 the production of poisonous aluminium ions, as will be mentioned later. 



Again, the reaction of the medium has a pronounced effect upon the 

 growth of bacteria, protozoa, and fungi. 



Gillespie and Hurst (1918) found that when grown in soils of acidity 

 pH 5-2 or more acid, potatoes were rarely affected with scab. Thus those 



