SOIL AS A MEDIUM FOR PLANT GROWTH 7 



soils and adds to their plasticity, and in addition, this very 

 fine material is an absorbent, holding the soluble plant-food 

 materials of fertilizers in a form that prevents them from 

 leaching from the soil, and yet gives them up to plants rather 

 easily. 



As examples of the second class we again have the feld- 

 spars as they furnish lime, magnesia and potash; calcite, 

 which contains lime ; hematite, which consists largely of 

 iron ; dolomite, which contains both lime and magnesia ; 

 apatite, which furnishes phosphoric acid and lime, and gyp- 

 sum, which is a combination of lime and sulfur. 



These minerals and the plant-food materials contained 

 in them may be reviewed in tabular form thus : 



As these minerals are widely distributed in rocks from 

 which soils are formed, they are found in almost all soils, 

 and thus it is that all the substances required by plants are 

 to be found in most soils. 



QUESTIONS 



1. What are the properties of soil that make it well adapted to 

 furnish a mechanical support for plants? 



2. What relation does soil have to the needs of plants for water? 



3. Describe the reasons why plants need water. 



4. Name the elemental substances that plants derive from soil. 



5. What elemental substance do plants obtain from soil that is 

 not present in rocks from which soil is formed? 



6. What two substances necessary to plant growth are contained 

 in the earth's crust in the smallest quantities ? 



