286 ROTATION OF CROPS. 



plant on tlie same soil, as there would not be potash 

 enough in it. 



752. If, however, the soil is left for some time 

 fallow, if no crop at all is raised on it, the soil has 

 time to renew itself; by the action of the air, the 

 further decomposition of the silicates and other simi- 

 lar stony compounds in the soil, is effected, and a 

 fresh supply of potash is provided. 



753. The same end is obtained by a system of ro- 

 tation. In place of sowing a second crop of the 

 plant which requires so much potash, some other 

 plant is taken, which does not require potash, but 

 which requires other substances that the soil con- 

 tains. 



754. In both these cases we restore, by the slow 

 effects of the air, those substances which we remove 

 with the first crop. In manuring, we at once add to 

 the soil that which the plants require. 



755. Generally speaking, wheat cannot be grown 

 on the same land year after year, and accordingly 

 various systems of rotation are adopted. It is 

 probable, however, that as soon as we know what are 

 the substances required by wheat and other crops, we 

 shall at once know how to restore to the soil those 

 substances which each crop removes, and thus be 

 enabled to obtain crops of the same plant, for any 

 number of years in succession, from the same soil. 

 The labors of chemists are, however, hardly far 

 enough advanced for this at present. 



756. The period of time over which the rotation 



