74 THE SOIL. 



uncultivated ground, and as the agencies ejQfecting the 

 decomposition of these ingredients and the transposition 

 of then- 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 portion 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 productive 

 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 sub- 

 stances 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 particles of their con- 

 stituents capable of affording nutrition. 



Wliat is here asserted of all the mineral nutritive sub- 

 stances witliout distinction applies equally to every soil 

 constituent required by the plant. The exhaustion of a 

 field may often simply depend upon a deficiency of 

 available silicic acid for the coming crop of cereal 

 plants, while the other food elements may be super- 

 abundant. 



It is evident from the nature of the process, that if 

 the soil is altogether deficient in disintegrable silicates or 

 soluble earthy phosphates, the action of time, the plough, 

 and the weather in fallow will not restore fertihty to a 

 field, and that the effect of disintegrating causes will vary 



