PHYSICAL PROPERTIES OF SOILS 23 



that only about one-half of the total that the soil will hold is 

 desirable at any one time. We shall learn something more 

 in another chapter about holding the water for the use of the 

 crops. 



Movements of the Plant Food. — We have said that the 

 plant gets from the soil all its food except carbon. We have 

 also said that the water in the soil is continually dissolving 

 small amounts of the soil grains and other chemical com- 

 pounds in the soil. All food for the plant must be in solu- 

 tion, that is, dissolved in the soil water. The carbon, as car- 

 bon dioxide, is a gas in the atmosphere and gets into the 

 plant through the pores on the leaves. The dissolved ma- 

 terials have a tendency to distribute themselves equally 

 through the soil water. The same thing happens if we put 

 a lump of salt in a vessel of water. Although we slip it down 

 in one corner of the vessel and do not stir the water it will not 

 be long before the water on the opposite side will taste salty. 

 The dissolved salt has spread throughout all the water in the 

 vessel. This is called diffusion. This diffusion and the 

 capillary movement of the water in the soil bring the dis- 

 solved material to the roots of the plant and the root hairs 

 near the tip of each rootlet take in the plant food and pass it 

 on through the roots, stem and branches of the plant to the 

 leaves, where it is worked up into various products such as the 

 plant uses to build up its structure and also to store away. 

 The root hairs are tiny thin- walled cells. The cell wall does 

 not have any openings in it that one has ever been able to 

 find, but still water and the dissolved materials pass through 

 it. This passage through a cell-wall, or membrane, is called 

 osmosis. It is a kind of diffusion. Inside the plant the 

 material has to pass from one cell to another by osmosis. 



