2 PLANT PRODUCTS 



of solar energy in the production of plant material lies 

 at the basis of all Agricultural Science and Practice. The 

 vegetable leaf in the plant is the prime mover which starts 

 a long chain of chemical change, which begins with the energy 

 derived from the sun and the crude materials brought 

 chiefly by the winds, and is supplemented by operations 

 and materials more under human control. 



For nearly all plant products we require — 



(i) The radiation from the sun. 



(2) A supply of water. 



(3) A supply of air. 



(4) A supply of fertilizers. 



(5) Correct conditions of heat, chemical reaction, and 

 bacterial development. 



In areas which are both tropical and continental the 

 sun's heat may be excessive for plant development, whilst 

 in polar regions the supply of solar heat is deficient ; but the 

 major part of the earth's surface receives enough heat for 

 ample plant life. 



In certain districts the amount of water may be excessive 

 and in other districts the reverse may be the case, but recent 

 study shows that these difficulties can be minimized if not 

 overcome. The supply of air to the leaf is usually sufficient, 

 but the supply of air to the roots of a plant very frequently 

 needs careful management to obtain the best result. 



Some soils are fairly well supplied b}^ nature with 

 appropriate fertilizers, but since the requirements of man 

 are very diverse, it is a virtual impossibility for a soil to be so 

 " fertile " that it needs no manure to produce the intensive 

 and varied crops which modern conditions may demand. 



Economic conditions may, however, prevent the produc- 

 tion of a maximum crop under intensive cultivation. It does 

 not always pay to produce maximum crops, and hence some 

 lands are said to be so fertile as not to need fertilizers. The 

 present war is teaching us that too much reliance may be 

 put upon the economic aspect of food production ; that 

 the interests of the nation are not identical with those of 

 the producer. 



