CHAPTER XII 

 PHOSPHORUS 



80. Presence in the Soil. Of all minerals necessary 

 for plant growth the compounds containing phosphorus 

 are most liable to be deficient. The average of many anal- 

 yses of the earth's crust shows the presence of only ^j 

 of 1 per cent of phosphorus, while many of our best arable 

 soils contain considerably less than that amount. The 

 phosphorus present in the soil is usually in the form of a 

 calcium phosphate. Calcium, with its valence of two, and 

 the phosphate radical, with its valence of three, unite in 

 accordance with the criss-cross rule stated previously, so 

 as to form normal calcium phosphate with the formula 

 Cas(PO)2. This is known in the trade as rock or bone 

 phosphate. There are also two acid phosphates, the di- 

 calcium phosphate Ca2H2(PO4)2, known as reverted phos- 

 phoric acid, and the mono-calcium phosphate, CaH4(PO4)2, 

 which, when mixed with calcium sulphate, is known as 

 the superphosphate of lime. The reaction with sulphuric 

 acid by which the insoluble rock phosphate is converted 

 into the soluble superphosphate is as follows: 



Should there not be enough sulphuric acid to complete 

 this reaction, or, in other words, should there be excess of 

 rock phosphate, the following reaction may take place : 



Thus, there will be formed the reverted phosphate, which 

 is insoluble in water. 



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