FOOD PHYSICALLY COMBINED. 83 



causes of this fact, so highly important to agriculture, 

 and which has been fully established by the experience 

 of several thousand years. If the reason be that cereal 

 plants feed 011 those substances only which are in physi- 

 cal combination in the surface soil, then we can easily 

 understand the remarkable fact of a field recovering its 

 power of production without any supply of manure ; for 

 though the nutriment in this form constitutes but a 

 small portion of the soil by weight, yet it imparts nutri- 

 tive qualities to a large volume of it ; and it is quite 

 intelligible that a soil not originally rich in nutritive 

 substances physically combined, when drained of them 

 by the innumerable underground absorptive organs of 

 a plant, must very speedily become unsuited lor the 

 cultivation of that plant. 



Now as the cultivated soil is composed in the main 

 of ingredients which are identical with the constituents 

 of uncultivated ground, and as the agencies affecting 

 the decomposition of these ingredients, and the trans- 

 position of their constituents affording food to plants 

 are in constant operation, it is easy to conceive how, by 

 the influence of such causes, an exhausted soil, which is 

 in fact nothing else than a soil reduced to its crude state 

 previous to cultivation, must regain the properties 

 which it had lost. With the conversion of a fresh por- 

 tion of the food elements from a state of chemical to 

 one of physical combination, the field recovers the 

 power of affording food to a fresh vegetation in such 

 quantity that the crops are again remunerative to the 

 agriculturist. 



An exhausted field which is again rendered produc- 

 tive by fallowing, may accordingly be defined as land 

 deficient in physically combined nutritive substances 

 necessary for a full crop, while containing an excess of 

 such substances in a chemically combined state. The 

 fallowing season, therefore, means the time in which 

 the nutritive substances pass over from the one state to 

 the other. It is not the amount of nutritive substances 

 that is increased in fallowing, but the number of parti- 

 cles of their constituents capable of affording nutrition. 



