FACTORS AFFECTING ABSORPTIVE POWER 293 



remaining ions are kept in the solution. There is a con- 

 stant tendency to bring the composition and density 

 of the solution into equilibrium, by diffusion and dios- 

 mosis, with the solution between the soil-particle and the 

 root-hair. The rapidity with which the metabolic pro- 

 cess removes a substance from the solution in the plant, 

 therefore, determines the rate at which it is removed 

 from a solution of given composition and density in the 

 soil. Plants making a rapid growth remove more nutri- 

 ents in a given time than those making a slower growth, 

 when the nutrient solution is of a given composition 

 and density. A maize plant, for instance, removes more 

 nutriment from a given solution in one day during its 

 stage of most rapid growth than does a wheat plant 

 during a corresponding stage. 



Another factor which affects the rate of absorption 

 of salts from the soil is the solvent influence of exudates 

 from the root-hairs. This subject has already been 

 treated (page 287), and it only remains to say that this 

 action apparently varies with different kinds of plants, 

 and probably accounts in no small measure for the dif- 

 ference in the ability of different plants to withdraw 

 salts from the soil. 



These several factors, which, when combined, deter- 

 mine the so-called "feeding-power" of the plant, are 

 recognized by the popular terms "weak-feeder" and 

 "strong-feeder," applied, on the one hand, to such 

 crops as wheat or onions, which require very careful 

 soil preparation and manuring, and, on the other hand, 

 to maize, oats or cabbage, which demand relatively 

 less care. In manuring and rotating crops, this difference 



