THE PHOSPHORUS GROUP OF FERTILIZERS 31 



over with dry, finely powdered mineral phosphate, which 

 prevents the sticky grains from cohering. At some works 

 the super-phosphate is dried and heated. In any case, 

 it is extremely important to produce a fine, dry powder, 

 which neither sticks to the hand in broadcasting, nor clogs 

 the drill in machine application. Super-phosphate should 

 always be kept in a dry situation, otherwise the skill and 

 labour of the manufacturer will be wasted (see p. 6). 



Super-phosphate, when stored, is apt to undergo a process 

 known as reversion, by which some of the soluble phosphate 

 once again becomes insoluble. The modern improvements 

 in manufacture have reduced the risk of depreciation in 

 value due to reversion during storage. Directly the super- 

 phosphate is applied to the land, reversion on a big scale 

 takes place. If the soil is tolerably well supplied with lime, 

 the mono-calcium phosphate will become converted into 

 di- or tri-calcium phosphate. When, however, the soil does 

 not contain very much lime, but is rich in iron, much of 

 the soluble phosphate will become ferric phosphate. The 

 former course of events is very much preferable. 



For the purpose of examining super-phosphate it is 

 common to take a portion that is soluble in water for the 

 estimation of phosphoric acid. In the United States of 

 America it is also common to determine the amount that 

 dissolves in ammonium citrate. The difference of the two 

 standards is not, in modern products, a great one. The 

 phosphate applied as super-phosphate will not penetrate any 

 depth in an ordinary soil beyond about six or eight inches. 

 Super - phosphate is of especial value as a quick-acting 

 phosphatic manure, and can be used even as a top dressing. 

 As many soils are deficient in phosphates, super-phosphate 

 is often one of the fertilizers which produce the most striking 

 and obvious results. 



A particular type of fertilizer which has proved useful 

 is called basic super-phosphate. This consists of a mixture 

 of super-phosphate and lime. By these means the super- 

 phosphate is turned into phosphate insoluble in water, but 

 very easily soluble in the very weakest of acids. (See Hughes, 



