94 



PLANT PRODUCTS 



that the total amount of phosphate in the crust of the earth 

 is not super-abundant for the purpose of wheat production. 



Table 17.- 



-Relative Supply and Demand of Elements in 

 Earth and Plants. 



There are many districts in the world which we know have 

 been cultivated for at least a few thousand years, but the 

 amount of phosphorus in the earth's crust, as shown in this 

 table, would only justify us in the conclusion that we could 

 grow bumper crops for 170 years. A commonly occurring 

 deficiency of phosphorus is, therefore, to be anticipated. 

 The other causes besides the lack of plant food are excess 

 or deficiency of moisture, indifferent physical condition, 

 absence of beneficial or presence of harmful soil organisms, or 

 the presence of some substances injurious to plant life. In 

 sandy soils the lack of colloids is so detrimental that almost 

 complete sterility may occur. The ordinary sand on the 

 sea-shore, for example, is very barren, owing to the action 

 of the sea water having washed out all colloidal material. 

 A few struggling plants may manage to make themselves 

 at home, and gradually add a certain amount of humus to 

 the soil, after which the general growth of plants may 

 begin. 



Dry Lands. — Land that is too dry and has too little 

 natural water is one that requires some system of irrigation 

 to be thoroughly satisfactory. Irrigation generally has to be 

 obtained by some reservoir system of water supply. Where 

 large rivers are obtainable, as in the northern part^^ of India, 

 and in the case of the Nile Valley, dams can be placed across 

 the rivers. On dry lands shallow tillage is essential. For 



