FOOD niYSICALLY COMBINED. ^^ 



chemical combination distribntcd tliroiigli tlic ground, is 

 also highly important, as serving to restore the state of 

 saturation, when the nutritive substances in physical com- 

 bination have been withdi'awn from the soil by a series of 

 crops reaped from it. 



Experience proves that the cultivation of deep-rooting 

 plants, which draw their food principally from the subsoil, 

 does not materially impair the fertihty of the surface soil 

 for a succeeding crop of cereal plants ; but the suc- 

 cessive cidtivation of the latter will, in a comparatively 

 small number of years, render the soil incapable of 

 yielding a remunerative crop. 



With most of our cultivated fields this state of exhaus- 

 tion is not permanent. If the ground is left fallow for 

 one or more years, especially if it is well ploughed and 

 harrowed during the time, it recovers the power of 

 yielduig a remunerative crop of cereal plants. 



Chemical analysis leaves altogether unexplained the 

 causes of this fact, so highly important to agriculture, and 

 wdiich has been fidly estabhshed by the experience of 

 several thousand years. If the reason be that cereal 

 plants feed on those sulDstances only which are in physical 

 combination in the surfiice soil, then we can easily under- 

 stand the remarkable fact of a field recovering its power 

 of production witliout any supply of manure ; for though 

 tlie nutriment in this form constitutes but a smaU portion 

 of tlie soil by w^eight, yet it imparts nutritive qualities to a 

 large volume of it ; and it is quite intelligible that a soil 

 not originally rich in nutritive substances physically com- 

 bined, when drained of them by the innumerable under- 

 ground absorptive organs of a plant, must very speedily 

 become unsuited for the cultivation of that plant. 



Now as the cultivated soil is composed in the main of 

 ingredients which are identical with the constituents of 



