The Calculations of Roberts 



133 



"As a clearer comprehension is had by comparing 

 unfamiliar things with familiar things, a table follows 

 which gives in brief the soil-exhaustion which is likely to 

 occur from a continuous twenty-year wheat-production. 

 Here, again, an average yield has been assumed which, 

 while approximately correct for New York, may be wide 

 of the mark in some states where the average yield of 

 wheat falls to eight or ten bushels an acre. 



"The following tables show the amounts and values of 

 the fertilizing ingredients removed by wheat (grain and 

 straw) in twenty years' continuous cropping, assuming 

 an average yield of fifteen bushels an acre and seven 

 pounds of straw to three pounds of grain: 



"COMPOSITION OF WHEAT AND STRAW 



'AMOUNTS AND VALUE OF PLANT-FOOD REMOVED IN ONE YEAR 

 AND IN TWENTY YEARS 



Total value in wheat, grain and straw for 20 years. . . .$128 .23 

 Total value in apple, fruit and leaves for 20 years 207 .45 



"The above table shows that the orchard requires, 

 if fruitful, plant-food equal in value to about $80 more 



