REMOVAL OF NUTRIENTS FROM THE SOIL 293 



may be a simple mechanical infiltration, although it is prob- 

 ably accompanied by a chemical reaction, or by a change in 

 the colloidal state of the membrane or both. Moreover, differ- 

 ent ions and molecules do not pass through the same cell-wall 1 

 with equal facility. Thus, one kind of ions may pass through 

 very readily while another kind may encounter extreme diffi- 

 culty in responding to diffusion tendencies. 



Differential diffusion may be ascribed to two conditions: 

 (1) different relationships between the cell-wall and the ions 

 and molecules of the entering material; and (2) differences 

 in the rate at which the entering molecules and ions are 

 utilized in the metabolic activities of the cell in particular and 

 the plant as a whole. The first case has been partially ex- 

 plained. If a compound ionizes into A and B ions and if A 

 ions, due to their relationship to the colloidal cell-wall, enter 

 more easily, a residue of B ions will be left in the soil solution. 



The second case may be illustrated by assuming the pres- 

 ence of potassium chloride in the soil solution. It ionizes 

 K + and CI" ions. Now conceive that these ions diffuse through 

 the cell-wall with equal facility in response to equilibrium 

 tendencies. If the potassium ions are used by the cell as 

 rapidly as they enter and are removed from solution, more 

 potassium will be absorbed. This might continue until the 

 potassium ions in the soil solution become much reduced in 

 number. If the chlorine, on the other hand, is but slightly 

 utilized by the plant, little will be drawn from the soil after 

 the initial equalization. Thus, a residue of chlorine might be 

 left from this type of differential absorption. This applica- 

 tion of diffusion principles shows the possibility, or even more, 

 the probability of plants leaving residues in the soil solution. 

 What the residues from different fertilizers may be and what 

 is the practical importance of such differential actions are 

 pertinent questions. 



1 The term cell-wall as used here refers to the cell-wall proper plus 

 the protoplasmic membrane. 



