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 Hopxrins and Pertir’s (34) table, while 
the amounts of sulphur removed are computed from Hart and 
PETERSON’S analyses. 
As pointed out by Heras and PErtit (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 
PouUNDS PER ACRE REMOVED 
NNUA 
Crop YIELD PER ACRE 
Sulphur Phosphorus 
Corn, cy ih Oe LE Or Cee es roo bushels 7.8 17.0 
SRNR MN oes a sin he vec Ca 100 bushels 5.8 11.0 
eee, Se 50 bushels 5% 12.0 
z Bo gear a ras BeBe eee at 11.4 9.0 
Ree PAPE 4 tons 13.0 20.0 
Pee AY oe. ic 8 tons 46.0 36.0 
WIMROCS oe i ee, 300 bushels 24.7 13.6 
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 TI 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 
