2OO 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



An example of the effect of dilution is presented below. 



Yields gm. dry matter 



i part per thousand 0-4934 



3 parts per thousand 0.7320 



5 parts per thousand i . 1540 



The plants were grown in water containing a mixture of the 

 various necessary salts. 



Solvent Action of Roots. Although plants can take up sub- 

 stances only in a state of solution, they have some power of 

 bringing substances into solution. Etchings showing the shape 

 of the root can be obtained by causing plants to grow upon 

 polished marble, and such etchings are often found in nature. 



The solvent action of roots is aided by the intimate contact 

 between root hairs, and soil particles, the latter often being 

 literally imbedded in the roots. The solvent action observed may 

 be brought about by the action of carbonic acid given off by the 

 roots, and the etchings mentioned above may be formed in this 



Fig. 47. A root hair, highly enlarged, showing the intimate 

 contact of root and soil. 



manner. According to Czapek, roots excrete potassium acid 

 phosphate, which has an acid reaction. The vegetable acids in 

 the root juices may also be effective, without actually passing 

 through the membrane. The vegetable acids are dissociated to a 

 certain extent into hydrogen and other ions ; for example, oxalic 

 acid may dissociate into the ions H and HC 2 O 4 . The ion HC 2 O 4 

 may be held in a non-diffusing condition, while the hydrogen is at 

 liberty to pass through the membrane, and thus exert an action 

 upon external substances. In this way there may be an exchange 

 of H and Ca ions, for example. Whatever the cause of the 

 solvent action of plant roots, it is well demonstrated that plants 

 can take up material not dissolved in the solution which extends 

 between the soil particles. 



