174 SOIL CONDITIONS AND PLANT GROWTH 



that the addition of soluble salts other than phosphates in- 

 creases the concentration of the soil solution. Much dis- 

 cussion has also arisen in regard to the supposed toxins : that 

 is dealt with on p. 244. 



As developed more recently by Cameron (65^), however, 

 the views of the American Soil Bureau have more in common 

 with those of the British workers. The solid particles of the 

 soil are supposed to consist of disintegrated rock minerals, 

 along with adsorption complexes, solid solutions such as the 

 so-called basic phosphates, and indeterminate substances in 

 an extremely fine state of division apparently containing 

 " humus," oxides of iron and aluminium, etc. The solubility 

 of these constituents in water is influenced by three circum- 

 stances : their fine state of division, the presence of CO 2 in the 

 soil water, and the fact that, as regards the solid solutions and 

 adsorption complexes, some sort of distribution coefficient 

 comes into play. Since the soil minerals are salts of strong 

 bases with weak and almost insoluble acids they become more 

 or less completely hydrolysed in solution, so that the con- 

 centration of the base would have to be very high before 

 equilibrium was attained. It is therefore improbable in 

 humid areas that equilibrium ever is attained. Moreover, 

 and this is a central part of the triesis, soil phenomena are 

 dynamic and not static ; the soil moisture, the soil solution, 

 even the very particles of the soil itself are in continual 

 motion and state of change. Hence the composition of the 

 soil solution must be continually changing. 



But it is considered that the amount of change in com- 

 position is small in comparison with the changes in the soil, 

 and such changes are not correlated with changes in the 

 productiveness of the soil. All the factors in soil fertility are 

 interdependent, and it is a mistake to confine attention ex- 

 clusively to any one aspect of their action. Thus fertilisers 

 should not be regarded exclusively as plant foods : they affect 

 more or less every soil factor influencing crop production, and 

 the problem can be satisfactorily solved only by discovering 

 the nature and extent of these interrelations. 



