182 Af/n'cuUural Gazette of X.S.W. [Mar. 2. 1920. 



Superphosphate and Rock Phosphate. 



Following the discovery of a rock phospliate a few years ago in New Soiitli 

 Wales, there has been much inquiry as to its effectiveness on different crops 

 as compared with superphosphate, and some experiments have been carried 

 out to determine its value as a fertiliser for maize in tliis State. 



Rock phosphate is the material horn which superphosphate is manufactured 

 by treatment with sulphuric acid. Different grades of it occur, but none 

 contain any water soluble phosphate. Ground to a fine state of division 

 (in which form it is sold as fertiliser) it contains about 20 to 30 ])er cent, 

 phosplioric acid soluble in strong acid, which is considered to be only sluwlv 

 available to plants. 



The late Dr. C. G. Hopkins, of the Illinois Agricultural Experiment Station,, 

 has been the champion of this fertiliser in the United States, and has aroused 

 there a good deal of criticism because of his contention that ground raw rock 

 })hosphate is the only fertiliser many farmers need buy. As the result of 

 many years" experiments at Illinois, he claimed that organic matter, such as 

 green manure, crop residue, and animal manure, need only be added to the 

 soil to make the phosphate from the insoluble ground rock available to plants 

 as food, the carbonic and other organic acids set free by the decay of the 

 organic matter taking the place of the sulphuric acid used by the manu- 

 facturers. He gives* the following points in favour of the use of raw rock 

 phosphate as compared with superphosphate : — 



1. Lower price per pound of phosphorus. 



2. Low grades of phosphate containing iron and alumina require larger 



use of sulphuric acid and also make an unsatisfactory product. 



3. It is free from acidity and has no tendency to injure the soil. 



4. It is present in all natural soil material, the phosphate being liberated 



for plant use by farmyard or animal manures and green manures 

 before the manufacture of superphosphate was ever thought of. 



A comparison of the prices of raw rock phosphate and super])hosphate in 

 America! shows the former in 1917 to have been less than half the price- 

 of the latter (rock phosphate 30s. per ton, superphosphate 70s. per ton), 

 though a little more than half the price in ]V)14. Even with this dis])arity 

 in price, the results of twelve years" experiments on maize show a larger net 

 gain from superphosjjhate, with but very little increased gain when animal 

 manure was also added, in our own State, while super])hosphate was quoted 

 at £5 per ton, rock })hosphate commanded £6 per ton. There seems no 

 reason why the latter should be dearer than superphosjthate, considering the 

 extra cost of treatment and the high ])rice of sulphuric acid at present. It 

 certainly seems a less valuable fertiliser for our conditions, and while it 

 remains at this price, or until it falls to somewhere near half the cost of super- 

 phosphate, it can scarcely be thought of. 



* " Soil Fertility and Permanent Agriculture," p. 242. 

 t Ohio M' n. Bull. 2, No. 12 (Decjmbor, 1917). 



