76 SOIL CONDITIONS AND PLANT GROWTH 



(in part), the swelling of the soil when wet and its shrinkage when dry, 

 are all colloidal phenomena. If we regard the mineral particles as the 

 skeleton of the soil we must look upon the colloids as clothing it in 

 many of its essential attributes. How the colloids are arranged in the 

 soil is not known, but the simplest view, and one in accordance with 

 all the facts, is that the mineral particles, especially the fine silicate 

 particles, are coated x with a colloidal complex containing silica, alu- 

 mina, ferric oxide, alkaline bases and phosphoric acid derived from 

 the weathering of the rock material and the so-called humus. These 

 various components are not in true chemical combination, but in a state 

 of absorption, or solid solution. The complex is decomposable by 

 changes in temperature, concentration of the soil solution, etc., but it 

 decomposes continuously and not in the per saltern manner of ordinary 

 chemical reactions. It can interact with various solutions, absorbing 

 certain substances as a whole e.g. organic dye stuffs or simply giving 

 up to the solution an amount of base equivalent to what it has absorbed. 



The study of the soil colloids is one of the most recent develop- 

 ments of the subject, and also one of the most promising. 



A wholly different conception of the constitution of the soil has 

 been put forward by Whitney and his colleagues of the Bureau of 

 Soils, United States Department of Agriculture (303-6). Soil particles 

 are supposed to arise by disintegration and to consist of the original 

 minerals of which the rock was composed ; little importance is attached 

 to the weathered silicates that play so large a part in the view just set 

 out. Colloidal properties and the special clay properties begin to ap- 

 pear when the disintegration has gone so far that the particles become 

 very minute : these properties are not associated with any particular 

 complex, but are supposed to be exhibited by any substance that is 

 sufficiently finely divided. Most agricultural soils arise from the same 

 minerals and are therefore of similar chemical constitution : in conse- 

 quence the solution in contact with the particles, i.e. the soil moisture, 

 is of similar composition and concentration for all soils. It is further 

 supposed that the concentration of any particular ion in the soil solu- 

 tion is not altered by addition of soluble salts, any such addition only 

 forcing out of the solution a number of the ions already there. 

 Special importance is attached to this soil solution and it is regarded 

 as the food of plants 2 and the source of fertility of the soil ; indeed the 

 function of the mineral part of the soil is mainly to hold up and distri- 



1 See also (88) and (89). 



2 It is interesting to note that a controversy on this point was going on fifty years ago 

 when agricultural chemists first began to use water cultures. See Schumacher, Landw. 

 VersHchs.-Stat., 1863, v., 270-307. 



