(id MAINE AGRICULTURAL EXPERIMENT STATION, 



mentioned — a part of this form may go into solution with the 

 citrate and be reckoned with the available. 



In the manufacture of superphosphates the conversion of 

 insoluble into soluble phosphates is never complete, a part being 

 unacted upon by the acid and remaining in the insoluble form. 

 Moreover, as wehave seen, a part of the soluble phosphate reverts, 

 especially on long standing. In practice, therefore, we always 

 find a superphosphate to consist of a mixture of the three forms 

 referred to. There must always be present sulphate of lime and 

 the impurities found in the original phosphate. 



Redonda Phosphate. — In another class of phosphates, not so 

 generally used, the phosphoric acid is combined with iron and 

 alumina, instead of with lime. These phosphates are not only 

 insoluble in water, and but very slightly soluble in hot ammonium 

 citrate, but they are even less available to the plant than the cor- 

 responding phosphates of lime. When treated with sulphuric 

 acid they prove very difficult of management, giving a pasty mass 

 which cannot be readily dried off. 



A phosphate of this description is quarried at Redonda, a 

 small island in the West Indies, and is known as Redonda phos- 

 phate or Redondite. It is a characteristic of this phosphate 

 that at a high temperature it loses water, and at the same time 

 becomes largely soluble in ammonium citrate. On long stand- 

 ing a reverse action takes place, the phosphate passing again to 

 the insoluble condition. It is probable that the reversion is 

 more rapid when the roasted Redonda has been applied to the 

 soil. Comparatively little of this phosphate is sold, yet on 

 account of the high percentage of phosphoric acid which it car- 

 ries and the ease with which it may be converted into the citrate 

 soluble condition, it would prove a valuable fertilizer if it is as 

 available to the plant as the chemical analysis would seem to 

 indicate. 



PHOSPHATES USED IN BOX EXPERIMENTS. 



In the experiments here recorded, three forms of phosphates 

 were used : 



I. Acid Florida rock. This was prepared by treating a 

 Florida phosphatic rock with sulphuric acid, thereby converting 

 a large part of the phosphate into an available form. At the 



