REVIEW AND CONCLUSION. 373 
that 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 
and 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 which 
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 pure 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, 
which 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 
