1922] 



WOODARD—SOIL FERTILITY 



99 



and phosphorus removed by some of the common crops. The 

 yields per acre and the amounts of phosphorus removed by these 

 yields are taken from Hopkins and Pettit's (34) table, while 

 the amounts of sulphur removed are computed from Hart and 

 Peterson's analyses. 



As pointed out by Hopkins and Pettit (34), these yields are 

 exceptionally large, but they have been obtained by some farmers, 

 and others may obtain them under proper systems of farming. If, 

 however, smaller yields are removed, it will not prevent soil deple- 

 tion, but will only delay soil exhaustion if the elements removed 



TABLE II 



Pounds per acre removed by farm crops 



Crop 



Com, grain . . . 

 Oats, grain . . . 

 Wheat, grain . 

 Timothy, hay. 

 Clover, hay. . 

 Alfalfa, hay. . 

 Potatoes 



are not returned in some form. In actual practice, failure to 

 return to the soil the elements of plant food which are removed in 

 the crops will result in a gradual decrease in yields, so that the 

 amounts of plant food removed will gradually become less. It is 

 impossible to determine the time when complete exhaustion will 

 take place, but a comparison of the amounts of plant food removed 

 by large crops with the amounts present in the soil will emphasize 

 the importance of renewing the supply in the soil before the soil 

 supply is reduced below that necessary for satisfactory crop yields. 

 Table III gives the pounds per acre of sulphur and phosphorus in 

 the surface soils analyzed and the number of years' supply of each 

 for several common farm crops, if maximum crops are removed, 

 such as are given in table II. 



Table III shows that all the soils are too low in sulphur to grow 

 alfalfa for 40 years, while 22 of them have phosphorus enough to 



