ECONOMIC PROPERTIES 



and the air, and (3) the mechanical structure of 

 the soil. The amount of rainfall and sunshine 

 remaining the same, the moisture and the ten> 

 perature of the soil, and its capacity for retain- 

 ing the chemical elements of fertility vary greatly 

 from place to place because of differences in the 

 size of the particles of the soil. By cultivation 

 the soil may be improved to some extent, in this 

 respect. By drainage and by irrigation the mois- 

 ture of the soil can be modified, and by the use 

 of glass and 'artificial heat the temperature of 

 both the soil and the atmosphere can be regulated. 

 But in most places and for most purposes Nature 

 has done infinitely more for man than he can 

 do for himself in providing the land with these 

 desirable physical qualities. 



From the standpoint of the economist the most 

 important chemical conditions of plant growth 

 are: (i) nitrogen, (2) phosphoric acid, (3) 

 potash, and (4) water. Other chemical com- 

 pounds contribute to plant growth, but these are 

 the ones which require our especial attention be- 

 cause they are present in the soil in limited and 

 varying quantities, and because they are more or 

 less readily exhausted and require considerable 

 effort to increase or replenish their supply. In 

 the humid regions where the water needed by 

 plants is abundantly supplied by Nature this ele- 

 ment of fertility requires little or no attention, 

 but in the arid regions water ranks first in eco- 



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