THE CONSTITUTION OF THE SOIL 79 



plant growth varied directly with the concentration. Again, the soil 

 solutions obtained from the different Broadbalk plots varied in com- 

 position in direct accordance with the fertiliser treatment and the history 

 of the plot. The comparative growth of plants in these solutions was 

 closely parallel to the growth of the crop on the plots, and corres- 

 ponded to the composition of the solution. This is in direct conflict 

 with the work of Cameron and Bell (69). 



As developed more recently by Cameron [68], 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 con- 

 sist of disintegrated rock minerals, along with adsorption complexes, 

 solid solutions such as the so-called basic phosphates, and indetermin- 

 ate substances in an extremely fine state of division apparently con- 

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

 these constituents in water is influenced by three circumstances : 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 concentration 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 thesis, 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 composition 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 exclusively to any one aspect of their action. Thus fertilisers 

 should not be regarded exclusively as plant foods : they affect more 

 or less every factor in the soil which influences crop production, and 

 the problem can only be satisfactorily solved by discovering the nature 

 and extent of these interrelations. 



