SECTION II. THE PHOSPHORUS GROUP 

 OF FERTILIZERS 



THE phosphorus group of fertilizers consists chiefly of 

 the following compounds : Mono-calcium phosphate, 

 CaH 4 P 2 O 8 .H 2 O, which is easily soluble in water, very 

 deliquescent, and strongly acid ; di-calcium phosphate, 

 Ca 2 H 2 P 2 O 8 4H 2 O, which is slightly soluble in water, and 

 is practically neutral to litmus paper ; tri-calcium phos- 

 phate, Ca 3 P 2 O 8 , a rather indefinite compound, much less 

 soluble in water, but attacked to some extent by carbonic 

 acid ; tetra-calcium phosphate, Ca 4 P 2 O 9 , which has been 

 found in basic slag ; apatite, Ca 5 (PO 4 ) 3 F, which is very 

 insoluble in water ; and some complex compounds, which are 

 both phosphate and silicate, occurring in basic slag. As, 

 with one exception, these materials are not very soluble in 

 water, it is necessary that most of the phosphatic fertilizers 

 should be very finely ground. In the case of basic slag the 

 commonly recognized standard of fineness is the ability to 

 pass a sieve containing 100 wires to the linear inch, or 10,000 

 meshes per square inch. This sieve is often used for other 

 fertilizers as well. Small experiments conducted at Cockle 

 Park, the Northumberland County Council experimental 

 farm, showed that this degree of fineness was about correct. 

 Those portions of phosphatic manures which only passed 

 sieves much coarser than the above had little influence 

 on the development of clover, whilst phosphatic manures, 

 which were so finely ground that they could pass a sieve with 

 200 wires to the inch, showed no appreciable advantage over 

 the standard. Special distributors have been constructed 

 to assist in spreading these manures over the land in an even 



