12 REVIEW OF PREVIOUS EXPERIMENTS 



of the question. Moreover many of the American State experiments, 

 e.g. Maine, Massachusetts, and Rhode Island, compare the two phos- 

 phates by applying equal money values, and it is obvious that such 

 trials have only a limited value as far as the application of the results 

 to this country is concerned. Moreover, changing economic conditions 

 must seriously detract from the value of their application to present 

 day American practice. The Ohio, Illinois, and certain of the Massa- 

 chusetts experiments compare equivalent quantities of the two forms 

 of phosphate, alone and in combination with other manures. These 

 experiments have extended through several rotations on duplicate, 

 and in some cases triplicate, plots. After an exhaustive review of 

 the American experiments up to 1908 Hopkins draws the conclusion 

 that rock phosphates are much the more economical type of phos- 

 phate to use, and that from the point of view of the permanent fertility 

 of the soil they are much to be preferred to acid phosphates. 



A later review of the American experiments is given by Waggaman 

 and Wagner(3i), covering the period up to 1917. These writers give a 

 table incorporating the results of 232 field experiments. Only 37 of 

 these experiments extended over a period of five years or more. Their 

 tabulation of these experiments is given in Table VI. 



In explanation of this table they give the following notes: 



Out of the 37 tests given in Table VI, 22 were carried on with a view to 

 comparing the relative merits of raw rock and acid phosphates. The conditions 

 under which such a comparison was attempted varied greatly, but it may 

 be said that in a general way, 13 of these experiments, or 59-1 %, gave crop 

 yields as favourable to raw rock as to the more soluble form of phosphoric 

 acid. Of the 9 experiments in which raw rock did not compare favourably 

 with acid phosphate, 2 were conducted on fields unresponsive to phosphate 

 treatments and 2 gave results which could be classed as either favourable or 

 unfavourable, depending on the method of interpretation employed. 



Of the 15 experiments in which no comparison between raw ground rock and 

 acid phosphate was attempted, 11, or 73-3 %, gave results strongly indicating 

 beneficial effects from the application of the former material, and 2 of the 

 remaining 4 experiments were conducted on fields showing little or no response 

 to phosphate treatment. 



In 21 experiments the applications of raw rock were relatively light (250 Ibs. 

 or less per acre), yet 15 of these experiments, or 71-4%, showed distinctly 

 favourable increases in yields on the fields treated with this material. 



In 16 experiments where the raw rock applications were more liberal, 13, 

 or 81-3 %, resulted favourably to raw rock phosphate, and the remaining 

 3 experiments were conducted on soils showing little or no response to phos- 

 phate treatment. 



Raw rock phosphate was applied in connection with organic matter in 23 

 experiments. Out of this number, 18, or 78-3 %, gave distinctly favourable 

 results, and of the 5 remaining experiments 3 were conducted on fields un- 

 responsive to other forms of phosphoric acid. 



