REVIEW AND CONCLUSION. 



^n 



tliat this native power of producing wheat will last unim- 

 paired for years, or, perhaps, centuries, provided the depth 

 of the soil is sufficient. In time, however, the silicates 

 atid other compounds whose disintegration supplies alka- 

 lies, phosphates, etc., must become relatively less in quan- 

 tity compared with the quite inert quartz and alumina- 

 silicates which cannot in any way feed plants. Then the 

 crop will fall off, and ultimately, if sufficient time be al- 

 lowed, the soil will be reduced to sterility. 



Other things being equal, this natural and durable pro- 

 ductive power is of course greatest in those soils whioh 

 contain and annually supply the largest proportions of 

 plant-food from their entire mass, those which to the great- 

 est extent originated from good soil-making materials. 



Soils formed from nearly [)ure quartz, from mere chalk, 

 or from serpentine (silicate of magnesia), are among those 

 least capable of maintaining a supply of food to crops. 

 These poor soils are often indeed fairly productive for a 

 few years when first cleared from the forests or marshes; 

 but this temporary fertility is due to a natural manuring, 

 the accumulation of vegetable remains on the surface, 

 M'hich contains but enough nutriment for a few crops and 

 wastes rapidly under tillage. 



Exhaustion of the Soil in the language of Practice has 

 a relative meaning, and signifies a reduction of producing 

 power below the point of remuneration. A soil is said to 

 be exhausted when the cost of cropping it is more than 

 the crops are worth. In this sense the idea is very indef- 

 inite since a soil may refuse to grow one crop and yet may 

 give good returns of another, and because a crop that re- 

 munerates in the vicinity of active demand for it, may be 

 worthless at a little distance, on account of difficulties of 

 transportation. The speedy and absolute exhaustion of a 

 soil once fertile, that has been so much discussed by spec- 

 ulative writers, is found in their writings only, and does 

 not exist in agriculture. A soil may be cropped below the 



