126 



FUNDAMENTALS OF FRUIT PRODUCTION 



hastens the change of toxic substances to harmless or beneficial materials, but it 

 is extremely important to the investigator for it strikes back to a fundamental 

 problem in plant nutrition." 



The whole question of the interrelationship of plants in the orchard 

 still needs thorough investigation. 



Antagonism. — Beside organic poisons, certain inorganic salts may 

 have toxic effects; for example, magnesium compounds may become 

 injurious to the higher plants. The toxic action of magnesium is modified, 

 however, by calcium because of the antagonism between these two ele- 

 ments. Salts of either calcium or magnesium by themselves tend to 

 increase the permeability of protoplasm more than a mixture of calcium 

 and magnesium salts in proper proportion. Therefore, the action of 

 calcium in offsetting the toxic effect of the magnesium probably is due 

 to diminished magnesium absorption when both elements are present in 

 suitable proportion. Antagonism occurs also between calcium and 

 potassium and many other salts. 



Aeration. — In the absence of aeration roots are unable to function prop- 

 erly and toxic substances are secreted. Moreover poor aeration favors 

 the formation of toxins by bacteria and in the absence of an adequate 

 supply of oxygen, numerous soil bacteria reduce nitrates, utilize the oxy- 

 gen and leave gaseous nitrogen which is not available to the higher plants. 



The physical character of the soil has an important effect on aeration; 

 stiff, retentive clays, for example, do not drain as well as sandy soils; 

 consequently they are usually not so well aerated. The application of 

 Hme or organic fertilizers to such clays may render them mellow, better 

 drained and more readily cultivated. 



Selective Absorption.— Within certain limits, plants are able to 

 absorb larger amounts of one mineral constituent at their disposal than 

 of another and in this way to exert a selective action. This is strikingly 

 shown by Table 15, which compares the percentage composition of the 

 ash of duckweed with the water in which it grew. 



Table 15. 



-Analyses of Ash of Duckweed and of the Mineral Matter Con- 

 tained IN THE Water in Which It Grew'"* 



This selective abihty of the plant may be explained by greater action 

 on certain constituents which are thereby rendered osmotically inactive 

 within the plant. This leads to further absorption of these particular 

 constituents. However, selective action has definite limits and plants 



