286 THE PRINCIPLES OF SOIL MANAGEMENT 



IV. ACQUISITION OF NUTRITIVE SALTS BY AGRICUL- 

 TURAL PLANTS 



All of the salts taken up by the roots of agricultural 

 plants are in solution when absorbed. The movement 

 into the root thus depends upon the presence of moisture, 

 which is the medium of transfer. The root hairs are the 

 great absorbing portions of the plant, and through the 

 cells of their delicate tissues the solutions of the various 

 salts pass by osmotic action. (See Fig. 53.) The nature 

 and quantity of material absorbed is determined by the 

 law of osmosis. From the cells of the root-hairs the dis- 

 solved salts are transferred to other portions of the plant, 

 where they undergo the metabolic processes that deter- 

 mine which constituents shall be retained in the tissues 

 of the plant. The unused ions which remain in the plant 

 juices prevent by their presence the further absorption 

 of those particular substances from the soil water. It thus 

 happens that the composition of the ash of a plant may 

 be very different from that of the substances presented 

 to it in solution. For instance, aluminum, although 

 always present in the soil, in a very slightly soluble form, 

 is either absent or present in mere traces in the ash of 

 most plants. On the other hand, iodine, although present 

 in sea-water only in the most minute amounts, is present 

 in large quantities in the ash of certain marine algae. 



147. Selective absorption. A plant will, in general, 

 take up more of a nutritive substance when presented 

 in large amount, as compared with the other soluble 

 substances in the nutrient solution, than if presented 

 in small amount. Thus, the percentage of nitrogen in 



