32 AGRICULTURE 



food. Others, such as chlorine, do not seem necessary to plant 

 life; most plants flourish without them. Others, such as iron, 

 are so abundant that even the poorest soils are 

 well supplied. 



Elements sometimes Lacking. There are 

 three elements which are necessary for plant food 

 and which are sometimes lacking in the soil 

 or are found in forms which plants cannot use. 

 These three are potassium, phosphorus, and nitro- 

 gen. More rarely, calcium is lacking. 



If the soil lacks any one element, that one 

 must be supplied. Without it a crop 

 will not thrive, although all others 

 may be abundant. The case is like 

 that of a painter who is making a 

 picture and needs several colors. He 

 cannot paint his picture if you give 



r . The buckwheat plant on the left is 



him much blue paint and no yellow, supplied with every element needed 



when he needs some blue and some exce P t P tash ' the P lant on the 



right is supplied with potash also. 



yellow. So Nature must have all her 



materials, the elements needed for crop growing. 



Now, it is the farmer's aim to get from his soil the largest crop 

 possible at the least possible cost. In order to do this he must 

 know what his land needs and how to supply it in the most eco- 

 nomical way. It is here that chemistry comes to his aid, as will be 

 explained in the chapter on Soil Improvement. 



EXPERIMENT 



Take two ounces of pure sand and two ounces of rich wood mold. 

 Heat each over an alcohol lamp and weigh again. The organic matter 

 has been burned off ; what remains is inorganic matter derived from the 

 rock. What difference is there between the sand and the mold ? 



