FERTILITY AND HUMUS IN DRY AREAS 407 



strates that the supply of fertility in these soils is very 

 great rather than that it is inexhaustible. 



Every non-leguminous crop grown upon the soil 

 and sold from it carries away from the soil a certain 

 proportion of the elements of fertility essential to plant 

 growth. This means' that the residue left in the soil 

 and subsoil is reduced by the amount removed in the 

 crop grown. It follows, therefore, by logical sequence, 

 that in time the elements of fertility in the soil will be 

 so reduced that it will be rendered incapable of produc- 

 ing a profitable return. The time when this result will 

 be reached will depend., first, upon the crop grown, and, 

 second, upon the abundance of the crop yields. The 

 more abundant the crop yields, the more rapid will be 

 the depletion in the fertility. It has been found that 

 to grow a bushel of wheat and the straw that it takes 

 to produce it, removes 1^4 pounds of nitrogen from the 

 soil, 1 pound of phosphoric acid and 1 4-5 pounds of pot- 

 ash. To grow a 20-bushel crop will remove twenty 

 times as much of these elements ; hence, if both straw 

 and grain are removed, the fertility left in the soil is 

 reduced by the amounts as calculated. Such a heavy 

 drain on soil fertility in the absence of any fertilization 

 must in time so deplete the store of plant food in any 

 soil, howsoever rich it may be in plant food originally, 

 that it will cease to grow profitable crops. In certain 

 areas in California where grain crops were profitably 

 grown for a generation, during recent years they have 

 fallen below the limit of profitable production in the 

 absence of fertilization. Even though production by 

 such a system should be profitable for two or even three 

 generations, the day of reckoning will assuredly come 

 because of the low yields obtained. 



The loss of plant food is less rapid in dry than in 

 humid areas. This arises, first, from the smaller growth 

 of straw produced by semi-arid soils; second, by the 



