CHEMISTRY OF FARM PRACTICE 



the growing plant; these are carbon, hydrogen, oxygen, 

 nitrogen, phosphorus, sulphur, potassium, magnesium, cal- 

 cium and iron. Some of these elements are derived from 

 the atmosphere, some from water and some from the soil. 

 Most of the substances of plants consist of the elements, 



FIG. 1. Percentage of elements in the combined mass of atmosphere, 

 waters and crust of the earth. 



carbon, hydrogen, and oxygen, which are obtained from 

 the atmosphere, or from water. Nitrogen is the most 

 expensive and the most elusive of the elements required 

 by plants. A part of the nitrogen may be derived from 

 the atmosphere by certain plants under conditions we 

 shall study later, but most of the nitrogen which serves as 



