78 SOIL CONDITIONS AND PLANT GROWTH 



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 of the Bureau of Soils, United 

 States Department of Agriculture, Washington (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, under similar climatic conditions, the concentration 

 of any particular ion in the soil solution is not materially 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 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. But the view that it is unalterable in composition has led to 

 some highly controversial deductions. In particular, soluble fertilisers 

 like potassium salts are not supposed to increase the amount of food 

 available to the plant, but to owe their beneficial effects to indirect 

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

 facilitation of movements of soil water, etc. 



Hall, Brenchley and Underwood (123*:) have repeated some of the 

 fundamental experiments, but have obtained results wholly different 

 from those of the American investigators. Breazeale's experiments 

 (51) are always quoted by the Bureau of Soils as proof that small 

 variations in concentration of the nutrient medium are without effect on 

 plant growth. The Rothamsted workers on the other hand found that 



1 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. 

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



