USE OF PHOSPHORUS IN DIFFERENT FORMS 287 



produced, and to meet the needs of such crops the applications of 

 plant food are made three times as large as in field experiments. 

 The soil used was from the gray silt loam prairie of the lower 

 Illinoisan glaciation, and wheat was the crop grown in the pots. 

 The phosphate used is known as the Tennessee blue rock. In cer- 

 tain pots the phosphorus was turned under with a good growth of 

 clover; in other pots with farm manure, and in others with both 

 clover and manure. 



TABLE 51. COMPARATIVE EFFECTS OF STEAMED BONE MEAL AND RAW ROCK 

 PHOSPHATE, IN CONNECTION WITH CLOVER AND MANURE 



It will be seen that the untreated soil (pot 101) yielded 10 grams 

 of wheat. Where clover was 'turned under (102), the yield was in- 

 creased by 6.3 grams, and where bone meal was turned under with 

 clover (105), the yield was 22.2 grams, the increase of 12.2 grams 

 being nearly double that produced by clover without bone meal. 



Where raw rock phosphate was turned under with clover (106), 

 the wheat yielded 23.3 grams, making a total increase of 13.3 grams 

 over the yield of the untreated soil. Of this increase 6.3 grams 

 should be credited to the clover and 7 grams to the rock phosphate, 

 by one computation; or 4.2 to the phosphate and 9.1 to the clover, 

 by the other route. Thus, rock phosphate used alone produced an 



