THE SOIL AND THE PLANT 139 



organic matter ; but the position is still maintained with 

 regard to the phosphoric acid, potassium, and calcium. 



The constancy in composition of the soil solution is readily 

 accounted for. Most soils arise from a relatively small number 

 of minerals. It is supposed that the process is mainly one 

 of disintegration and not decomposition. The large frag- 

 ments of the soil are shown to consist of unaltered minerals 

 and the small fragments could probably all be identified if 

 suitable methods were available. Thus most soils are chemically 

 alike from the qualitative point of view ; they are mixtures of 

 the same complex minerals and differ only in the proportions 

 of the mixture and the size of the fragments. In consequence 

 the soil solution in equilibrium with these particles has the 

 same composition in all cases. Further, it is argued that the 

 addition of a soluble potassium salt to the soil cannot increase 

 the concentration of the solution in K ions, since any of the 

 new ions going into solution would simply throw others out 

 of solution. The solubility product, it is supposed, must remain 

 approximately constant whatever soluble salts are added. 



The mineral particles of the soil constitute the reserve from 

 which the soil solution is reinforced as fast as soluble material 

 passes into the plant. But they serve another purpose ; they 

 form the great framework over which the soil solution arranges 

 itself in accordance with the laws governing surface attractions. 

 Any local disturbance brought about by the action of plants 

 or other causes is readjusted at a speed which depends on 

 the size and nature of the particles, the presence of organic 

 matter, and various other factors. Whilst therefore all soils 

 are alike in regard to the composition of the soil solution they 

 show great differences in respect to the arrangement of the 

 solution over their particles and the speed of readjustment; 

 in other words, in the speed at which the nutrient solvent is 

 supplied to the plant. 



The fertility of the soil does not depend on the chemical 

 composition of the soil for two very good reasons : the com- 

 position of the soil solution is the same in all soils ; and even 

 if it were not variations in composition would be without 

 effect on plant growth. In most soils fertility depends on 

 the physical factors regulating the supply of the soil solution 

 to the plant. 



But this simple view of the constitution of the soil must 



