356 



(5) Light for the synthesis or building up, in the 

 leaves, of organic compounds from the inorganic elements 

 of plant food. 



(6) Air which not only supplies a large part of the 

 food of plants, but which in small quantity in the soil 



Fig. 106. Cattle on the Range in Northern Saskatchewan. 



provides a desirable environment for plant roots and at 

 the same time plays a large part in the development of 

 "available" plant food. 



Fortunately for us nature provides most of these re- 

 quirements with a lavish hand. It remains for man to 

 increase, or at least maintain the supply of those that 

 under his own climatic and soil conditions have been 

 either wasted or but sparingly supplied. 



312. Light and Air Essential, but of little Practical 

 Importance. Light and air are as important to the plant 

 as plant food, water or heat, yet neither is as important 

 to the crop grower for the reason that he finds both pro- 

 vided in very great abundance. More light than reaches 

 the earth in most climates where agriculture is practised, 

 is seldom needed, and even if it were, the amount could 

 not be increased profitably. 



