272 NATURE STUDY AND AGRICULTURE 



water. It has such an affinity for oxygen that the moment 

 it is exposed to the air it begins to unite with oxygen. 

 Place a small piece, not larger than a sweet-pea seed, on a 

 porcelain dish and watch it. It slowly unites with the 

 oxygen of the air. Care should be taken not to touch 

 phosphorus with the finger, or to leave it where it will 

 touch anything that will burn. 



Phosphoric acid may be obtained by burning a bone, 

 pulverizing it, and placing it in a bottle with a weak solution 

 of sulphuric acid. The lime of the bone and the sulphuric 

 acid unite and leave the clear liquid, the phosphoric acid. 



Now make a collection of as many of the elements and 

 compounds used by plants as you can get. 



Some time should be taken at this point for the dis- 

 cussion of what is meant by fertility of the soil. Of the 

 ten elements that all plants must have, seven are supplied 

 by the soil. Name these seven. Where do the plants get 

 oxygen, hydrogen, and carbon? If any of the elements 

 supplied by the soil are lacking or are not in an available 

 form, we say the soil is poor, or is lacking in fertility. There 

 are many physical conditions as well as plant materials that 

 help to make soil fertile. These will be discussed in a 

 later lesson. But all the proper physical conditions known 

 will not make a fertile soil if it lacks any of the elements 

 that plants must have in order to manufacture their plant 

 products. 



Most of the substances used by plants exist in such 

 abundance in the soil that there is little danger of their 

 becoming exhausted. Three elements, however, that all 

 farm crops use are not so abundant. These are phos- 

 phorus, potassium, and nitrogen. In some places calcium 



