TBE AGRICULTURAL JOURNAL. 



745 



Agricultural Chemistry for Beginners. 



CHAPTER VL 



By Archibald Pearce. 

 THE PHOSPHATES. 



Superphosphates. 



LTHOUGH it was shown that by 

 means of acids the insoluble phos- I 



phates can be brought into solution, this 

 is not an example of the simple kind of 

 solution that takes place when sugar dis- 

 solves in tea, for instance, or salt in 

 water, where no chemical change occurs ; 

 in reality the acid seizes on some of the j 

 calcium, and the phosphate becomes con- 

 verted into the mono-calcic variety. This 

 is the principle upon which the so-called 

 superphosphate is made. The tri-calcic 

 phosphate, usually in the form of mineral 

 phosphate but sometimes as bones, animal 

 charcoal (which is only charred bone), or 

 guano, is ground to a fine flour, and 

 treated with the proper proportion of sul- 

 phuric acid ; the hydrogen of the acid 

 and two portions of the calcium of the 

 phosphate change places, and the result : 

 is mono-calcic phosphate and calcium I 

 sulphate (sulphate of lime) ; and when , 

 properly dried we have the well-known I 

 superphosphate of commerce. When made i 

 from bones, it is called dissolved bones ; 

 and when partly of bones and partly of 

 mineral phosphate, dissolved bone com- 

 pound ; but even dissolved bones are 

 generally made with a small proportion 

 of mineral phosphate, as pure bone super- 

 phosphate is apt to be pasty and difficult 

 to dry, or the difficulty is avoided by 

 only converting a portion of the bone 

 into superphosphate. Most of the mixed 

 or " special " manures have superphos- 

 phate as their groundwork or basis, other 

 ingredients boing added to suit the special 

 needs of the crop for which they are 

 manufactured. The great , value of this 

 product lies in its solubility, which makes 

 it the quickest acting of all phosphatic 

 manures ; the rain carries it about in the 

 soil, and so wherever the roots penetrate ■ 

 they find their food ready for them. It is 

 especially suited for soils rich in lime, 



and on such it is probably the best phos- 

 phatic fertiliser. 



Di-CALCic Phosphate, 



If superphosphate is brought into con- 

 tact with lime it is converted into the di- 

 calcic form, which pure water will not 

 dissolve. It is, however,, readily soluble 

 in very weak acids, and in consequence 

 is probably just as easily taken up by 

 plants as the soluble form ; but it has to 

 a great extent lost its power of diffusing 

 through the soil. Also when superphos- 

 phate is kept in stock for some time, the 

 amount of mono-calcic phosphate con- 

 tained in it is found to gradually diminish, 

 a certain proportion of di-calcic phos- 

 phate being formed in its place. This 

 latter is, therefore, often called reduced 

 or reverted phosphate, and the manure is 

 said to go back. Practically there is 

 little if any loss of value through this 

 action ; in fact, farmers sometimes pro- 

 duce the reduction artifically. The re- 

 duced phosphate is much less liable to be 

 washed away from light soils than the 

 more soluble superphosphate, and so it is 

 easy to see that in some casee it is prefer- 

 able. A useful method of reducing 

 superphosphate is to mix it with about 

 one-third its weight of bone dust, and 

 moisten the mass slightly, turning it over 

 occasionally for five or six weeks. In this 

 way some of the bone phosphate reacts 

 on the superphosphate, and both are con- 

 verted into the reduced state. Such a 

 mixture is often better than either of its 

 ingredients on soils with little lime in 

 them. Instead of bone-dust we may use 

 the same weight of basic slag ; a thorough 

 mixing must be given, and the manure 

 can be used at once. When an analyst, 

 in reporting on a manure, speaks of 

 citrate-soluble phosphate, he is referring 

 to the di-calcic variety, a weak solution of 

 citric acid being the nearest way yet de- 



