26 THE SOIL OF TnE FARM. 



cropping, without manuring, is to reduce the stock of 

 available fertility in the soil. But since it is the minimum 

 of any one essential ingredient and not the maximum of 

 the others which is the measure of fertility, it follows 

 that a soil which is exhausted for one plant may still 

 contain an abundant food suj^ply for a plant of another 

 kind. A rotation of crops will in such case defer the 

 period of exhaustion. But whatever the crops cultivated, 

 it is plain that continued croppmg without the use of 

 manures must ultimately bring us to a time when the 

 crops grown will no longer pay the cost of cultivation. 



The particular substance on which the crops grown 

 have made the largest demands, and which was originally 

 most deficient in the soil, will be the first to become ex- 

 hausted. Further, the more available substances will be 

 removed while the less soluble will remain behind; a poor 

 soil will be reduced to sterility sooner than a rich one ; a 

 shallow soil will fail sooner than a deep one ; and a light 

 soil sooner than a stiff one. All soils, however, are ca- 

 pable of yielding annually from their stores of natural 

 fertility a certain amount of produce, and this constant 

 abstraction of their substance is not necessarily incon- 

 sistent with the maintenance of fertility ; for, indepen- 

 dently of the small quantity of vegetable food, so to 

 speak, available for use at any one time, an immense 

 store resides in most soils in a dormant condition, capable 

 of gradual development as it is required. 



Restoration of Fertility, — As cropping removes these 

 substances from the soil, they are replaced more or less 

 rapidly and completely by the agencies of the weather. 

 The action of earth-worms is also useful m this respect, 

 for it is as soil-fertilizers rather than as soil-formers that 

 earth-worms are of imiiortance to auriculture. Indeed, 

 they are rarely met with in soils that are very destitute 

 of organic matter. The richer the soil, however, and 



