142 



PEACTICAL FARM CHEMISTRY. 



foods, approximately, are taken off each acre of 

 ground, viz. : 



A considerable portion of the nitrogen, however, 

 was drawn from the air by the agency of the clover; 

 another, smaller, part probably was washed down 

 from the atmosphere in rains. The exact amount 

 of these outside contributions, however, cannot be 

 determined, and we must be contented with a very 

 rough estimate. 



Suppose that the nitrogen from these sources 

 amounts to not much more than fifty pounds, the 

 soil is yet called on, for the satisfaction of the de- 

 mands of the five crops, to furnish 296 pounds of 

 nitrogen, 118 pounds of phosphoric acid, and 299 

 pounds of potash. Thirty tons of average good 

 barnyard manure contains about 300 pounds of ni- 

 trogen, 120 pounds of phosphoric acid, and 300 

 pounds of potash — or almost exactly the amount 

 needed for the production of the liYe crops. Such 

 manure application (an aggregate of thirty tons of 

 stable manure per acre during a period of five years) 

 can not be called excessive. It is practiced by many 

 farmers, and seldom barren of the most satisfactory 

 results. In most cases it will not only maintain, 

 but actually improve the original soil fertility. If 



