64 



ILLINOIS ACADEMY OF SCIENCE 



with the final assumption of the acid character by the soil is 

 due solely to the acids set free by the flocculation of organic 

 and inorganic suspensoids of the soil. There are a great num- 

 ber of acid forming processes going on in the soil; carbon di- 

 oxide production of the respiration and fermentation of soil 

 organisms and nitric acid production by the nitrifying pro- 

 cesses (oxidation of ammonia salts to nitric acid by nitrosom- 

 onas and nitrobacter) may be mentioned as two important 

 ones. 



As one passes from the coarse sands to the finer clay he finds 

 a continual rise in the specific surface presented by the soil — 

 that is, a continual rise in the total surface of the soil par- 

 ticles per unit volume. The high specific surface of the finer 

 soils is due in part to particles of micronic or microscopic 

 size, but in large part to the ultramicroscopic or submicronic 

 particles — soil colloids, inorganic and organic. The great 

 specific surface developed in fine soils, especially those with 

 considerable colloidal constituents brings about much adsorp- 

 tion or concentration of dissolved and suspended substances on 

 the surface of the soil particles. This surface accumulation is 

 due in part to electrical effects typical of suspensoids and in 

 part to the surface tension active substances in accordance with 

 the Gibbs-Thompson law. Whatever the cause of these sur- 

 face concentrations the result is that many new reactions are 

 set up between the concentrated materials themselves and be- 

 tween them and the soil particles. We often speak of sandy 

 soils being poor in plant nutrients, while clays are rich. This 

 is largely due to the surface phenomena and the reactions re- 

 sulting from them. 



In fact any good soil shows many of the characters of a 

 colloidal medium, or is essentially a colloid, as is well expressed 

 by a quotation from Russell's Monograph (31) on Soil Con- 

 ditions and Plant Growth, "It is a mistake to suppose — and 

 this point cannot be too strongly emphasized — that the medi- 

 um on which the soil organisms live, and which is in contact 

 with the plant roots, is the inert mineral matter that forms 

 the bulk of the soil. Instead, the medium is the colloidal com- 

 plex of organic and inorganic compounds usually more or less 

 saturated with water, that envelops the mineral particles. It 

 is therefore analogous to the plates of nutrient jelly used by 

 bacteriologists, while the mineral particles serve mainly to sup- 

 port the medium and control the supply of air and water and, 

 to some extent, the temperature." 



