14 REVIEW OF PREVIOUS EXPERIMENTS 



In regard to the cumulative effect of raw ground phosphate rock it may be 

 said that in 17 instances (46 % of the entire number of experiments) there 

 was evidence of greater availability after raw rock had been applied for a 

 number of years. In 13 out of the remaining 20 experiments the data are not 

 sufficient to give evidence on this point, and in 4 out of the 7 cases where no 

 cumulative effect was shown the soils were not responsive to phosphate treat- 

 ments. 



The same writers (quoted in the Scottish Journal of Agriculture 1 ) 

 in a survey of the above experiments conclude that: 



To be efficacious as a fertiliser rock phosphate must be spread evenly over 

 the ground as a fine powder. The presence of decomposing organic matter 

 increases the efficacy, probably because of the greater bacterial activity pro- 

 duced and the higher percentage of carbon dioxide given off. Fineness of 

 the powder and the presence of organic matter together prolong the efficacy 

 of raw phosphate rock for another year, or even more. On the other hand, 

 as the action of superphosphate is more rapid than that of bone powder, 

 basic slag and mineral phosphates, it is probably preferable to any other 

 phosphatic fertiliser when the aim is to obtain rapid growth of the plants 

 cultivated. 



To obtain the best results with powdered rock phosphates, they must be 

 applied in larger quantities than superphosphate. Whether it be best to apply 

 rock phosphates in a soluble or insoluble form to produce the most economical 

 increase in yield depends on the nature of the soil, the cultural method, the 

 price of the phosphates, the duration of the vegetative period, and other local 

 factors. It is a question which, to a certain extent, must be solved by each 

 farmer individually. 



French Experiments. Grandeau, in the seventh volume of his 

 Etudes Agronomique, gives an account of French experiments with 

 dissolved and undissolved phosphates on potatoes, wheat and oats. 

 The experiments are reviewed by Dyer (8), who records that "contrary 

 to generally accepted theories (but conformably with results already 

 arrived at in various parts of France) finely powdered mineral phos- 

 phates have given yields as large as those of superphosphate the 

 soil being an extremely poor non-calcareous one." 



English Experiments. The earliest experiment on the value of 

 rock phosphates was that carried out by Dr Daubeny, on the turnip 

 crop, with Spanish phosphorite (5), at the Botanic Gardens at Oxford. 

 Very satisfactory returns were obtained from the Spanish phos- 

 phorite, which, however, did slightly better when treated with sul- 

 phuric acid. In the same issue of the Journal Sir H. Verney, Bart.(SO) 

 gives an account of his experiments on barley with this material. 

 The experiments were carried out on a heavy sandy loam. In these 

 trials Spanish phosphorite gave quite as good results as superphosphate 



1 July 1920, p. 357. 



