95 



Superphosphates, or soluble phosphates, are derived from the insolu- 

 ble materials already described by first grinding to a powder and then 

 mixing with sulphuric acid, which changes the insoluble phosphoric acid 

 to the soluble form. The soluble phosphoric acid thus obtained is a de- 

 finite chemical compound, and is identical in composition whatever may 

 have been the material from which it was derived. The term super- 

 phosphates is, therefore, applied to any material containing soluble phos- 

 phoric acid as its chief constituent. In superphosphates there is nearly 

 always present, however, in addition to the soluble, the reverted form, 

 which is probable quite as useful as the soluble form. The superphos- 

 phates made from boneblack and bone ash differ from the mineral super- 

 phosphates niainly in showing a higher content of " available" phospho- 

 ric acid, an average of 16 per cent., which is practically all soluble. 

 Mineral superphosphates contain on the average 14 per cent, of available 

 which may include from 1 to 3 per cent, of reverted, besides more or less 

 of the insoluble. Superphosphates made from animal bone differ from 

 those made from the other materials mentioned in containing nitrogen 

 in addition to phosphoric acid. They are, however, sometimes called 

 " ammoniated superphosphates" or " dissolved ammoniated bone." 



In the use of phosphoric acid, therefore, it must be remembered 

 that the source has an important bearing in determining whe- 

 ther it is used as a phosphate or as a superphosphate. As regards the 

 untreated phosphates, it must be remembered that those derived from 

 organic substances, such as bone, are the more valuable because of their 

 greater tendency to decay and greater ease of solution, and that this 

 tendency to decay is promoted by such means as will increase the fine- 

 ness of division. In the case of superphosphates, those which contain 

 the greatest proportion of soluble phosphoric acid are relatively the 

 most valuable, because the soluble phosphoric acid readily distributes it- 

 self in the soil and goes to the roots of plants, while the reverted re- 

 mains where it is placed and the roots of the plants must come to it. In 

 the next place, it should be remembered that phosphoric acid is not 

 washed from the soil, though in a soluble form, since it is finally " fixed" 

 by coming in contact with lime, iron, and other mineral substances 

 usually present in good soils. 



POTASH. 



Potash may exist in a number of forms, though chiefly as chlorids, or 

 muriates, in which case the potash is combined with chlorin ; and as 

 sulphates in which the potash is combined with sulphuric acid. With 

 potash, however, the form does not exert so great an influence upon 

 availability as is the case with nitrogen and phosphoric acid. All forms 

 are freely soluble in water, and are believed to be nearly if not quite 

 equally available as food. The form of the potash has, however, an im- 

 portant influence upon the quality of certain crops, due rather to the 

 constituents with which the potash is associated than to the potash itself. 

 For example, it has been demonstrated that the quality of tobacco, 

 potatoes, and certain other crops is unfavourably influenced by the use 

 of muriate of potash, while the same crops show a superior quality if 

 materials free from chlorids have been used as the source of potash 

 applied. 



The chief sources of potash salts at the present time are the Stass- 

 furt mines of Germany, and the products of these mines, which are 



