BIOLOGY OF AGRICULTURAL PRODUCTION 101 



the past with exhausted and abandoned soils, the people 011 

 these rich lands are again talking of the " inexhaustible f er- 

 lility" of the soil, and burning their straw and manure or 

 hauling the latter onto the ice to befoul their streams. A 

 comparison of virgin soil in the Canadian "Northwest with soil 

 adjoining it which had been cultivated twenty-two years 

 showed a loss of nitrogen per acre from 6936 to 4736 pounds, 

 or 2200 pounds, a loss of $3-50.00 worth of nitrogen per acre. 



FERTILITY CONTAINED IN DIFFERENT FARM CROPS 

 (Approximate maximum amounts removable per acre annually) 



1 Much of this nitrogen is taken from the air, and the roots go so deep that 

 even the phosphorus and potassium may be largely supplied from layers of 

 soil below the reach of other crops. 



