INFLUENCE OF ABSORPTION ON SOLUBILITY 279 



extract, as the tendency to maintain equilibrium of 

 the solution overcomes the absorptive action. With the 

 removal of the adsorbed substances, the equilibrium 

 between the soil particles and the surrounding solution 

 is disturbed, solvent action is increased, and more 

 material gradually passes from the soil into the solution. 

 In this way the uniform and continuous body of extrac- 

 tives is maintained. 



142. Other factors. For purposes of soil analysis, 

 the quantity of water used for extraction must be placed 

 at some arbitrary figure, and the method is open to the 

 objection that it does not represent accurately the soil 

 water solution. Analyses of soils of different types are 

 not comparable, and the water extract cannot be con- 

 sidered to measure the concentration or even the com- 

 position of the solution existing between the root hair 

 and the soil particles. However, for studying some of 

 the changes that go on in the soil, and which are detect- 

 able in the soil-water solution, the method may be used 

 to advantage. 



III. MINERAL SUBSTANCES ABSORBED BY PLANTS 



The plant, in its process of growth, withdraws from 

 the soil certain mineral matters that are presented to 

 its roots in a dissolved condition. As the salts in solution 

 are quite numerous, and as the osmotic process by which 

 the absorption is accomplished does not admit of the 

 entire exclusion of any substance capable of diosmosis, 

 there are to be found in the plant most of the mineral 

 constituents of the soil. Some of these are concerned in 



