REVIEW OF PREVIOUS EXPERIMENTS 11 



more insoluble than those phosphates which are readily dissolved in 

 dilute solutions of citric acid. In view of these results it is reasonable 

 to expect that promising returns would be secured from similar trials 

 with less resistant materials like rock phosphates. 



Burlison(3) made an elaborate series of pot experiments with six 

 types of rock phosphate, the trials extending over a period of three 

 and a half years and embracing the results from 700 pot cultures. 

 It is difficult to interpret the exact meaning of these experiments in 

 terms of basic slag or superphosphate as neither of these forms of 

 phosphatic fertilisers was included in the trials. The results from 

 this elaborate series are nevertheless of considerable interest as they 

 show that the phosphates in rock phosphates, even of the hard re- 

 sistant type like Canadian Apatite, can be assimilated by farm crops 

 in sand cultures under greenhouse conditions and in the absence of 

 decaying organic matter. Three other conclusions from his work are 

 worth noting. Burlison found that the plants could obtain their 

 calcium as well as their phosphorus from rock phosphates, and that 

 the addition of calcium carbonate to the rock phosphates did not 

 produce better results. An attempt was made to ascertain the effect 

 of fineness of grinding on the availability of such phosphates, and 

 the work shows that better results were secured by grinding beyond 

 the '100' grade. Finally the author gives it as his opinion that 

 there is no particular relation between the citric acid, soluble phos- 

 phoric acid and the availability of rock phosphates to the plant. 



These pot experiments, scanty and incomplete though they may 

 be, agree in demonstrating that, under the conditions of the experi- 

 ments, the insoluble phosphates in fluorspar basic slag and in rock 

 phosphates may have a very considerable agricultural value. 



American Experiments. Although by no means exhaustive, a 

 large number of field experiments have been carried out with rock 

 phosphates. The subject has perhaps received more attention in 

 the United States than elsewhere. There considerable differences 

 of opinion exist concerning the fertilising value of raw ground rock 

 phosphates or 'floats.' In the States the controversy centres round 

 the relative value of ground rock phosphates (floats) and acid phos- 

 phate (superphosphate). Most of the American experiments, a detailed 

 account of which is given by Hopkins (ii), are confined to this aspect 



