FALLOWING. ' 285 



cessary to the fertility of the land, is removed. It 

 remains for us to inquire, what are the best means 

 of restoring these matters to the soil ; and likewise, 

 in how far it is possible to increase the fertility of 

 the soil, by augmenting the natural quantity of any 

 of its constituents. 



749. In practice there are three courses adopted 

 to maintain the fertility of the soil ; namely, fallow- 

 ing, a rotation of crops, and the application of 

 manure. 



750. In order to comprehend the eifect produced 

 by fallowing, it is necessary to remember the mode 

 in which soil is formed. As has already been stated, 

 soils consist principally of small fragments of differ- 

 ent rocks and stones, which, from a variety of causes, 

 have gradually crumbled down to powder. The sand 

 or fine particles of stone in the soil are constantly 

 decomposing (649) and adding to the soil, potash, 

 soda, and very finely divided earthy matters ; it is 

 from sources of this kind that most of the saline sub- 

 stances present in the soil, are derived. 



751. The soluble salts formed in this manner, are 

 dissolved by the rains, and in great part washed 

 away from the surface ; a portion, however, always 

 remains in the soil, and is absorbed by plants. When 

 a crop of some plant, requiring, for example, a large 

 quantity of potash, is raised in such a soil, it often 

 happens that the crop takes away nearly all the solu- 

 ble potash the soil contains ; and in consequence, it 

 would be impossible to raise a second crop of that 



