. THE COLLOIDAL PROPERTIES OF SOIL 173 



ticular complex, but are supposed to be exhibited by any sub- 

 stance that is sufficiently finely divided. Most agricultural 

 soils arise from the same minerals and are therefore of similar 

 chemical constitution : in consequence, the solution in contact 

 with the particles, i.e. the soil moisture, is of similar composi- 

 tion and concentration for all soils. It is further supposed that, 

 under similar climatic conditions, the concentration of any par- 

 ticular ion in the soil solution is not materially altered by addi- 

 tion of soluble salts, any such addition only forcing out of the 

 solution a number of the ions already there. Special import- 

 ance is attached to this soil solution and it is regarded as the 

 food of plants l and the source of fertility of the soil ; indeed, 

 the function of the mineral part of the soil is mainly to hold 

 up and distribute this solution. Soluble fertilisers, like potas- 

 sium salts, are not supposed to increase the amount of food 

 available to the plant, but to owe their beneficial effects to in- 

 direct actions in the soil, such as the precipitation of toxic 

 substances, facilitation of movements of soil water, etc. 



Controversy has arisen in three directions. It was at first 

 supposed that the physical properties of the soil moisture 

 were in the main deducible from the hypothesis that the 

 water was suspended as on grains of sand or like oil on 

 bullets (see p. 166). This has been shown to be inadequate ; 

 it is necessary to take the soil colloids into account. 



The view that the soil solution is unalterable in composi- 

 tion led to considerable discussion. Hall, Brenchley, and 

 Underwood (i2i</) and F. H. King (147^) obtained diametri- 

 cally opposite results to those of Whitney* and Cameron. 2 It 

 is now generally recognised that the composition of the soil 

 moisture varies with the soil and the treatment, and, indeed, 

 that the changes in composition are on the whole related to 

 the fertility of the soil (see p. 125). Bouyoucos (49^) finds 



1 It is interesting to note that a controversy on this point was going on sixty 

 years ago when agricultural chemists first began to use water cultures. See 

 Schumacher, Landw. Versuchs-Stat., 1863, v., 270-307. 



2 For bibliography of this discussion see Stewart, Jcurn. Ag. Res., 1918, 

 12, 311-368. 



