410 SCIENCE PROGRESS 



" apparent acidit}'-," " real acidity," " potential acidity," 

 " absorption acidity," " lime requirements " of soils and of 

 plants, " immediate " and " continuous " lime requirements, 

 " active " and " latent " lime requirements, etc. ; such a 

 multiplicity of terms tends only towards confusion and obscurity. 



Theories of Soil Acidity 

 Some soils are apparently so acid that when moist they will 

 redden litmus paper almost immediately, but their aqueous 

 extracts seldom redden litmus after boiling off the COg. The 

 comparatively few cases reported in which the COg-free aqueous 

 extracts were acid to litmus have invariably been the result of 

 highly abnormal conditions, and in such cases the immediate 

 cause of the reaction is obvious. Most acid soils, however, 

 yield so little soluble acid on extraction with water alone that it 

 cannot usually be detected by litmus paper after boiling off COj,. 



(A) Humic Acid Theory 

 Various theories have been put forward at different times 

 to explain such acidity as this. Sprengel in 1826 attributed 

 the acidity to the accumulation of insoluble complex organic 

 acids — the so-called humic acids — produced by the decomposi- 

 tion of plant residues left over from the crop. The dark alkaline 

 solution obtained on treating an acid soil with ammonia was 

 supposed to contain the soluble ammonium salts of these acids 

 and the acids themselves could be precipitated on acidifying. 

 Such acids were also supposed to occur in neutral and alkaline 

 soils combined with calcium or magnesium, and are then 

 practically insoluble in alkalies without a previous extraction 

 with acid. This hypothesis long held the field. The compounds 

 obtained from soils, however, were very indefinite and variable 

 in composition and always contained mineral impurities that 

 could not be eliminated. As analytical methods improved it 

 was realised that these differences were real differences in 

 composition and were not, due merely to imperfections in the 

 analytical methods, and doubt arose as to whether these so-called 

 humic acids really were definite chemical compounds. 



(B) Selective Adsorption Theory 



The first real advance was due to van Bemmelen ' in 1888, 

 who considered that these bodies were not definite chemical 

 compounds but absorption complexes, i.e. mixtures of a base 

 and of various colloidal substances held together by some sort 

 of surface attraction. These ideas were further developed into 

 a general theory of soil acidity by Baumann and Gully about 

 1 9 10. The acid reaction of peat moss and of peat soils was 



^ For detailed references to literature, see Journ. Agric. Sci., xi {1921), p. 42. 



