7 6 AGRICULTURE. [CH. 



shown that, under certain circumstances, corn can be 

 successfully and profitably grown upon land, year after 

 year, for a long period of time. This, however, admits 

 of a ready explanation. If you refer to the quantities 

 of inorganic matter removed from the soil (41) you 

 will see that wheat makes a large demand upon the 

 soil for silica ; but when you compare the quantities of 

 phosphoric acid, lime, potash, and soda removed by 

 the several crops, you will observe very great differ- 

 ences between that which is necessary for the wheat 

 crop, and that required for the other crops there re- 

 ferred to. For instance, a crop of beans takes nearly 

 double the phosphoric acid required for wheat, and a 

 turnip crop takes three times the quantity. A crop of 

 beans takes more than three times as much lime as a 

 crop of wheat, and a crop of turnips and clover each 

 takes ten times as much. So also with potash, beans 

 taking four times the quantity required by wheat, turnips 

 taking eight times as much, whilst clover takes double 

 the quantity ; and so also in the case of soda. With- 

 out laying too much stress upon the exact proportions 

 so drawn from the land, it is evident that, whilst wheat 

 makes a larger demand upon the soil for silica, com- 

 paratively speaking, it requires but a moderate supply 

 of other inorganic fertilizers. 



157. In fact, the continuous growth of wheat with- 

 draws from the land a constituent which occurs prac- 

 tically in an unlimited quantity. But it must be 

 remembered, that the successful conduct of this practice 

 necessitates that thorough cultivation of the soil, 

 which makes a large quantity of the silica existing in it 

 in a dormant condition, take an active form, and there- 

 by become available for the growth of the crop. A 

 supply of silica cannot be thus made available from 

 all soils, as for instance in the case of sandy soils, 

 although the silica exists in these in a very large 

 proportion. The soils upon which this result is obtain- 

 able, are those which contain a large supply of silica in 



