4i8 SCIENCE PROGRESS 



scientific, and only possible meaning, i.e. the hydrogen-ion 

 concentration of the nutrient solution bathing the soil particles. 

 And yet the hydrogen-ion concentration of the soil solution, 

 which is in equilibrium with the reserve of acidity bound up 

 with the soil mass, is one of the few soil factors susceptible of 

 investigation by really accurate methods. Until comparatively 

 recently soil acidity has been studied as though it were entirely 

 unrelated to the ordinary physico-chemical concept of acidity ; 

 the reason being that the practical agricultural problem involved 

 is a complex one requiring for its complete solution considerations 

 based on colloid chemistry and physics as well as considerations 

 of acidity in the physico-chemical sense. But a complex 

 question such as this cannot be solved by any general method, 

 and the attempt to do so has been the main defect in most of 

 the earlier work on this subject. The only rational way of 

 attacking such a problem would appear to be to resolve it into 

 its various factors and study each factor separately — in so far 

 as our present experimental methods permit. One of these 

 factors is the true acidity, in the physico-chemical sense, of the 

 soil solution — that is, acidity regarded as a function of the 

 hydrogen-ion concentration. This, of course, is not the whole 

 story — it is only one chapter of it — but it is known to be a funda- 

 mental one, and is besides one of the few soil properties capable 

 of study by exact quantitative methods. Soil acidity, in this 

 narrower sense, thus becomes a phase of the broader question 

 of soil reaction in general, and its study should be merged in 

 that of the more comprehensive question. This broadening of 

 the point of view is also justified from agricultural considerations. 

 Soil acidity is generally regarded as a pathological condition of 

 the soil which may, and should, be removed by liming. This 

 is not universally the case, however. In potato growing in 

 particular an acid condition of the soil is beneficial, and is, in 

 fact, generally preferred, because a certain degree of acidity * 

 is not only not injurious to the potato crop, but is inimical to 

 the organism causing soft scab — Actinomyces chromogenus — one 

 of the worst of potato pests. This disease, however, never 

 appears on a soil of a certain degree of acidity. This has met 

 with some response in agricultural practice, and in this case 

 obviously acidity is a desirable condition. 



Moreover, large acid areas occur — particularly in America — 

 where, owing to altitude, transport difficulties, or other causes, 

 liming is a difficult and expensive and often an impossible 

 operation, and in such cases attempts have been made to 

 develop a definite system of acid land agriculture that appears 

 to have met with some success. 



1 Expressed_by V^ = 5 or less. 



