114 



PLANT FOOD 



72. Plant Foods. About eighty distinct elements l are 

 known to the chemist. Thirteen of these are used by 

 plants. The following table names these thirteen and 

 gives some facts about them. 



Nitrogen 

 Phosphorus 

 Potassium 

 Calcium 



Chlorine 



Iron 



'Magnesium 



Silicon 



Sodium 



Sulphur 



Nitrogen 

 Carbon 

 Hydrogen 

 Oxygen 



These four elements are supplied by the 

 soil, but soil is likely to be deficient in 

 one or more of them, and then the farmer 

 must supply the need. 



These six elements also are supplied by the 

 soil. None of these elements need be sup- 

 2>lied by farmers. 



These four elements are supplied by the 

 air and the water. They are always pres- 

 ent if the soil is not lacking in water. A 

 combination of carbon and oxygen forms 

 the gas called carbon dioxide ; and a com- 

 bination of hydrogen and oxygen forms 

 water. 



The farmer, then, needs only to attend to the supply 

 of soil water and of the four elements in the first group 

 above. The supply of soil water has been discussed, 

 briefly, and the matter will receive more attention later. 

 The supply of calcium, the main substance in lime, and 

 one of the four important elements, will be treated in the 

 next chapter. The present chapter will now deal with 

 the other three elements, nitrogen, phosphorus, and 

 potassium. These are sometimes called the three great 

 agricultural elements. 



1 An element is a substance which cannot be resolved into different kinds 

 of substances. Air was once called an element, but it is no longer called so, 

 because chemists have found how to separate it into different gases. Each of 

 these gases, however, like oxygen or nitrogen, is an element. 



