MOTHER EARTH 93 



69. The composition of organisms. Another way of finding 

 out what there is in the soil that the plant depends upon is to 

 make an analysis of the plant to see of what it is composed. 

 Analysis shows that certain elements are present in the plant 

 body; some of these elements are also present in the soil. It is 

 therefore reasonable to suppose that the plant derives these 

 elements from the soil. It does not follow, however, that every- 

 thing taken by the plant from the soil is of use to the plant. 

 The most common elements found in plants are the following : 



Carbon Sulfur Potassium 



Oxygen Phosphorus Sodium 



Hydrogen Calcium Iron 



Nitrogen Magnesium Chlorin 



Compare these with the composition of the human body 

 (Fig. 3) to see how much we are like the plants. Chemical 

 studies show that all animals have a composition differing very 

 little from that of the human body. 



The materials taken from the soil by the growing plant are sometimes 

 called plant food. Strictly speaking, these are not food, as we shall see 

 later (see Chapter X) ; they are merely some of the materials out of 

 which plants manufacture their food. 



70. Exhaustion of soil. After many crops of plants are re- 

 moved the soil lacks some of the mineral salts required for plant 

 growth. There is no danger of exhausting the iron in a soil, 

 for this element is used in such small quantities that plants will 

 have stopped growing for lack of some of the other elements 

 (for example, phosphorus or nitrogen) long before the iron 

 supply is considerably reduced. In some soils the same may 

 be true of calcium. But the other elements are used in such 

 large quantities (in proportion to the quantities present in 

 most soils) that they practically limit the use of soil for crop 

 raising. To make up for the withdrawal of materials by crops 

 it has for ages been customary to put on or into the soil various 

 substances called fertilizers. These include limestone or gyp- 

 sum, barnyard manure, guano, crushed bones, ground phosphate 



