CHEMISTRY OF SOILS 133 



in a crystalline or an amorphous state. They are insoluble 

 in water, and more or less difficultly soluble in dilute acids 

 according to the strength, temperature, etc. It is pos- 

 sible that in many cases the tricalcic phosphates do not 

 undergo any chemical changes but gradually become more 

 readily available to plants merely as a result of mechanical 

 pulverisation. Any transient acidity of the soil would tend 

 to convert the tricalcic phosphate into the soluble mono- 

 calcic, CaH 4 (P0 4 ) 2 ;form. The phosphate, however, cannot 

 long continue to exist in that state ; on contact with lime 

 it is reprecipitated as dicalcic or tricalcic phosphate 



CaH 4 (P0 4 ) 2 + CaC0 3 = 2 CaHP0 4 + C0 2 + H 2 



Monocalcic Dicalcic 



phosphate. phosphate. 



CaH 4 (P0 4 ) 2 + 2 CaC0 3 = Ca 3 (P0 4 ) 2 + 2 C0 2 -+ 2H 2 



Monocalcic Tricalcic 



phosphate. phosphate. 



On contact with hydrates of iron or alumina this soluble 

 phosphate would be precipitated as phosphates of these 

 bases. 



3 CaH 4 (P0 4 ) 2 + 4 Fe (OH) 3 = 4 Fe P0 4 + Ca 8 (P0 4 ) 2 +12 H 2 



Monocalcic Ferric Tricalcic 



phosphate. phosphate. phosphate. 



3 CaH 4 (P0 4 ) 2 + 4 Al (OH) 3 = 4 Al P0 4 + Ca 3 (P0 4 ) 2 + 12 H 2 



Precipitated phosphate of lime is much more readily 

 soluble in dilute acids than the crystalline mineral, and 

 its solubility is probably not greatly diminished on drying. 

 The phosphates of iron and alumina, even in the gelatinous 

 hydrated condition in which they appear when freshly 

 precipitated, are insoluble in dilute acetic and some other 

 acids, but are probably available to plants. They are 

 rendered much more difficultly soluble on drying, and 

 probably they become gradually non-available. It is very 

 desirable, therefore, that soluble phosphates should be 



