85 



capabilities of a soil are in question, for only those 

 parts of a soil which exist in physical combination 

 are available as plant-food, and it follows that the 

 fertility of a soil will be proportionate to the 

 amount of this portion. 



It is quite immaterial, for the immediate require- 

 ments of husbandry, how much potash, or phosphoric 

 acid, or lime a soil may contain, if these substances 

 are present only in chemical combination, in which 

 condition they are, as it were, locked up, and can- 

 not be assimilated by the plant. A large store of 

 these substances is necessary to ensure the lasting 

 fertility of a soil ; but in the absence of sufficient 

 mineral plant-food in a state of physical combina- 

 tion, the soil will be as barren as pulverized rock. 



The advantage derived by allowing fields to 

 lie fallow for some time to recover their original 

 fertility, has been known to the husbandman from 

 the most remote times ; but the causes which effected 

 this change from sterility to fertility have, even in 

 the present century, been but imperfectly under- 

 stood. The late Baron von Liebig was the first 

 who gave a satisfactory explanation of this 

 phenomenon, by explaining that the improvement 

 of the soil by what is called fallowing, especially 

 when well ploughed during that time, is solely due 

 to the action of moisture, heat, and atmospheric 

 air, which decomposes the different constituent 

 parts of the soil, separates them from their chemical 



