SOIL MINERALS L65 



It occurs in hexagonal prisms frequently of very small 

 size, almost like needles, and green or red in color. 



This compound is practically insoluble in water but under 

 the action of water and carbon dioxide it slowly dissolves, 

 possibly according to the following equation: 



2Ca 5 (P0 4 )3CI + 12C0 2 + 12H 2 = 3CaH 4 (P04) 2 + 6CaH 2 (CO ? )2+ CaCl 2 . 



Most of the phosphorus in soils, how r ever, occurs in an 

 amorphous, secondary form possibly derived from apatite, 

 expressed by the formula Ca 3 (P04)2 and called tri-calcimn 

 phosphate. This compound in the presence of small amounts 

 of carbon dioxide in water changes to the dicalcium phos- 

 phate, thus: 



Ca 3 (P04) 2 + 2C02+ 2H2O = Ca 2 H 2 (P04)2+ CaH2(C0 3 ) 2 . 



In the presence of more carbon dioxide and water it decom- 

 poses to the monocalcium phosphate, thus: 



Ca 3 (P04) 2 +4C02+ 4H 2 = CaH 4 (P04)2+ 2CaH 2 (CO?)2. 



Tricalcium phosphate is practically insoluble in pure water, 

 1 part requiring 50,000 parts of water, whereas dicalcium 

 phosphate is soluble to the extent of 1 part in 7500 parts of 

 water, and monocalcium phosphate, 1 part in 100 of water, 



Soil water, of course, is never pure water, having always 

 some substances dissolved in it, and these substances in 

 solution tend to modify the above solubilities to a slight 

 extent; but the figures serve to show the relative solubilities 

 of the three forms of phosphate. Dicalcium phosphate and 

 monocalcium phosphate, both of them, can be used by plants. 

 Since carbon dioxide is so generally prevalent in soil water 

 and can dissolve tricalcium phosphate, it is safe to say 

 that all compounds of phosphorus with calcium are available 

 forms of phosphorus for plant use. 



Phosphorus also occurs as more or less indefinite com- 

 pounds of iron phosphate, FePCX, and aluminium phosphate, 

 AIPO4, both very insoluble in water, or in water and carbon 

 dioxide, or in any other ordinary soil solution, unless calcium 



