46 DRY FARMING 



divided into three groups for the purposes of our dis- 

 cussion, are as follows : 



Group 1. Group 2. Group 3. 



Carbon Calcium Nitrogen 



Hydrogen Magnesium Phosphorus 



Oxygen Iron Potassium 



Sulphur 



Carbon, Hydrogen, Oxygen. Carbon is derived from 

 the carbon-dioxide of the air. By means of their 

 '^breathing pores" the leaves of plants take in this gas 

 and in some way combine it with water to make sugars. 

 Oxygen is likewise derived from the air, both as carbon- 

 dioxide and free oxygen gas, and from water, of which 

 it is a constituent part. Hydrogen is derived from 

 water. These three elements, carbon, hydrogen and 

 oxygen make up from 90 to 95% of mature plants and 

 this fact, together with the fact already noted that they 

 are derived from the atmosphere and water, has led to 

 the error of minimizing the importance of the other seven 

 essential elements. 



Calcium, Magnesium, Iron, Sulphur. Of the remain- 

 ing seven essential elements, four, calcium, magnesium, 

 iron and sulphur, though necessary to plants, occur in 

 relatively large amounts in the soil in proportion to the 

 amounts required by plants. Therefore they do not con- 

 cern us, since they are not likely to limit our crop yield. 

 The soil is the source of these elements for plants (some 

 sulphur comes down with the rain). 



Nitrogen, Phorphorus, Potassium. The elements of 

 this group, nitrogen, phosphorus and potassium, are, for 

 common farm crops, derived from the soil. (Legumes 

 can take nitrogen from the air, as will be discussed 

 later). Potassium occurs in liberal amounts in most 



