82 THE SOIL. 



which oppose their distribution, or their disintegration 

 and decomposition. The perennial plants, and particu- 

 larly the so-called weeds, consuming in proportion to 

 the time less food, and absorbing longer, will always 

 thrive on a soil of this description long before annual 

 or summer plants, which in their shorter period of 

 vegetation require a far larger amount of nutritive sub- 

 stances for their full development. 



The longer a soil is under cultivation, the more it 

 becomes suited for the growth of summer plants, from 

 the recurrence and operation of the causes by which 

 the nutritive substances are converted from a state of 

 chemical into one of physical combination. To be pro- 

 ductive, in the fullest sense of the term, a soil must be 

 able to afford food at all points in contact with the roots 

 of the plants ; and, however small the quantity of this 

 food may be, it must necessarily be distributed through 

 every part of the soil. 



The power of the soil to nourish cultivated plants is 

 therefore in exact proportion to the quantity of nutritive 

 substances which it contains in a state of physical satii- 

 ration. The quantity of the other elements in a state 

 of chemical combination distributed through the ground 

 is also highly important, as serving to restore the state 

 of saturation when the nutritive substances in physical 

 combination have been withdrawn from the soil by a 

 series of crops reaped from it. 



Experience proves that the cultivation of deep-root- 

 ing plants, which draw their food principally from the 

 subsoil, does not materially impair the fertility of the 

 surface soil for a succeeding crop of cereal plants ; but 

 the successive cultivation of the latter will, in a com- 

 paratively small number of years, render the soil incapa- 

 ble of yielding a remunerative crop. 



With most of our cultivated fields this state of ex- 

 haustion 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 

 yielding a remunerative crop of cereal plants. 



Chemical analysis leaves altogether unexplained the 



