SOME CONCEPTIONS OF THE SOIL 9 



of the soil acts as a plant nutrient in itself and functions 

 also as a solvent for other materials. By its circulation it 

 not only promotes solution but it continually brings nutrient 

 elements in contact with the absorbing surfaces of the roots. 

 The two prime functions of the soil are thus realized through 

 the factors discussed above — mechanical support and a suffi- 

 cient supply of certain nutrient elements under favorable 

 conditions. 



7. Nutrient elements. 1 — Although the physical condition 

 of the soil exerts a far-reaching influence on plant growth, 

 the relationships involved are more readily understood than 

 those which have to do with plant nutrition. Moreover, the 

 solubility of the necessary nutrients is very closely related 

 to the complex processes of soil formation. Ten elements 2 

 are usually considered as necessary for plant growth. If one 

 is lacking, normal development will not occur. They may 

 be classified as follow^ :• 



From air or water From the soil 



Carbon Nitrogen Calcium 



Oxygen Phosphorus Magnesium 



Hydrogen Potassium , Sulfur 



Nitrogen Iron 



Plants obtain most of their carbon and oxygen directly 

 from the air by photosynthesis and respiration. The hydro- 

 gen comes, at least partially, from water. All of the other 

 elements, except a small amount of nitrogen utilized directly 

 from the air by certain plants, are obtained from the soil. 

 It must not be inferred, however, that the bulk of the plant 



1 For an excellent discussion of the functions of plant nutrients, see 

 Russell, E. J., Soil Conditions and Plant Growth, Chap. II, pp. 30-46; 

 New York. 1915. 



* It may be possible that manganese and silicon and possibly chlorine 

 and fluorine function as nutrients. They as well as sodium, aluminum, 

 titanium, barium, strontium, and certain rarer elements are found in 

 plant ash. 



