The Plant World. 



size and aggregation of the soil particles. With the same soil, 

 the more water present, the lower will be the rate of oxygen 

 movement. Thus, if we increase the water supply beyond a 

 certain limit, we often cause injury to the plants through lack 

 of oxygen to the roots. This consideration can hardly be 

 overemphasized. 



The rate of possible supply of inorganic food materials to 

 plant roots is a function of the amount of soluble materials 

 present in the soil and of the concentration of the solution, as 

 the latter is determined by the amount of water at hand. It is 

 to be remembered in this connection that the plant usually 

 absorbs these substances as ions and not as salts. As ions are 

 removed from the soil solution others of like nature take their 

 places, dissolving away from the soil particles themselves. 

 The stream of water entering the roots is probably always far 

 more rapid than is necessary to keep the plant supplied with 

 the non-nitrogenous food constituents. Therefore, since most 

 soils contain the non-nitrogenous salts necessary to plant life, 

 the rate of diffusion of ions or salts is probably seldom, if ever, 

 a limiting condition for plant grow T th, excepting in the case of 

 nitrogenous compounds. These latter salts are perhaps to be 

 considered the most important of all the inorganic food materials. 

 They are formed and destroyed by bacterial growth in the soil, 

 their amount depends upon the bacterial flora, upon the amount 

 of organic matter present, upon temperature, and probably upon 

 other factors. The rate of supply of nitrates and similar bodies 

 is surely often a limiting condition to the extent or rapidity of 

 plant growth. 



In some localities the soil conditions are adverse to plant 

 life, or to the activities of all but certain peculiar forms, through 

 the presence in the soil of poisons. This is notably the case in 

 bogs or moors and, from present indications, also in many upland 

 soils. Such poisons seem usually to be organic bodies resulting 

 from the growth of organisms, such as bacteria, fungi and even 

 the roots of higher plants themselves, but may, of course, be 

 inorganic in certain areas. 



Finally, the soils of the arid regions sometimes contain such 

 large quantities of soluble inorganic salts that the concentration 

 of the soil solution is physically very great and thus the entrance 

 of water into the roots is completely or all but prevented. In 



