CHAPTER VI 



THE ELEMENTS NEEDED BY PLANTS 



Source of the elements. The chemical elements indispensable 

 to plants are ten in number ; namely, oxygen, hydrogen, nitro- 

 gen, potassium, magnesium, calcium, iron, sulphur, phosphorus, 

 and carbon. Several others have been found in plants, but 

 these have been proved unnecessary by growing plants to 

 maturity in media in which these elements were lacking. Al- 

 though the ten elements indicated are all essential, most of 

 them are taken in very minute quantities. The small amount 

 of ashes left when wood is burned represents the mineral 

 matter taken up by the plant in forming it, and much of this 

 is likely to be silicon and other elements that are nonessen- 

 tial. Very little is known of the part played by some of the 

 essential minerals in the economy of the plant. Possibly their 

 presence acts simply as a stimulant for various plant processes. 

 The only element taken entirely from the air is carbon. Oxy- 

 gen is taken from the air for respiration, but that used in 

 making food is taken combined with hydrogen as soil water. 

 All the other elements are derived from the soil, in which they 

 exist as compounds and not as single elements. These com- 

 pounds are usually chlorides, carbonates, sulphates, phos- 

 phates, and nitrates. In most of these there is considerable 

 oxygen, the termination ate in the names of the compounds 

 indicating its presence. These materials are dissolved out of 

 the soil by the soil water and carried into the plant by osmosis. 



Selective absorption. Analysis of the ash of plants has shown 

 that all the species of a given area do not contain the same 

 proportion of the different minerals, though growing under 



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