116 BULLETIN 699, U. S. DEPARTMENT OF AGRICULTURE. 



of carbon dioxide, is a potent factor in rendering rock phosphate 

 available to crops. 



The advantage of pot and greenhouse work lies in the fact that 

 by the exercise of the greatest care the conditions under which the 

 crops are grown may be, to a large extent, controlled. 



The final proof of the value of raw rock phosphate as a fertilizer 

 must rest upon field experiments. A large amount of field work has 

 been conducted with this material under a variety of conditions, but 

 unless the numerous factors influencing crop yields be taken into con- 

 sideration the results appear very conflicting. In considering the 

 results of field experiments, careful attention must be given to the 

 following factors: (1) Uniformity of experiment field, (2) topog- 

 raphy and drainage conditions, (3) physical and chemical composi- 

 tion of the soil, (4) previous treatment of the field, (5) climatic con- 

 ditions, (6) injuries from disease, insects, and animals, (7) kind of 

 crops grown and selection of seed, (8) rate of application and uni- 

 form distribution of phosphates, (9) methods of comparing raw rock 

 with other phosphates, (10) effect of other fertilizers, (11) number 

 and distribution of plots, (12) duration of the experiment. 



Two hundred and thirty-two field experiments with raw rock phos- 

 phate have been reported by the State experiment stations. Of this 

 number only the 37 experiments given detailed consideration in this 

 bulletin were conducted for a period of five years or longer. A sum- 

 mary of these 37 field tests is given in Table LXVIII, but in view of 

 the fact that nearly all of these tests were conducted under different 

 conditions and involve the consideration of variable factors, it is ob- 

 viously impossible to classify the data in such a way as to give each 

 experiment its proper weight. The classification given in the table 

 therefore is in some instances necessarily arbitrary. 



Out of the 37 tests given in Table LXVIII, 22 were carried on 

 with a view to comparing the relative merits of raw rock and acid 

 phosphate. The conditions under which such a comparison was at- 

 tempted varied greatly, but it may be said that in a general way 13 

 of these experiments or 59.1 per cent gave crop yields as favorable to 

 raw rock as to the more soluble form of phosphoric acid. Of the 

 9 experiments in which raw rock did not compare favorably with 

 acid phosphate, 2 were conducted on fields unresponsive to phosphate 

 treatments, and 2 gave results which could be classed as either favor- 

 able or unfavorable, depending on the method of interpretation 

 employed. 



Of the 15 experiments in which no comparison between ground raw 

 rock and acid phosphate was attempted, 11 or 73.3 per cent gave 

 results strongly indicating beneficial effects from the applications 

 of the former material, and 2 of the remaining 4 experiments were 



