368 SOILS: FEOPEETIES AND MANAGEMENT 



with this zeohte — which is an amorphous gel containing 

 potassium, calcium, aluminium and silicic acid — found 

 that there was no absorption of phosphoric acid from a 

 neutralized solution of monocalcium phosphate or from 

 a solution of dicalcium phosphate at various degrees of 

 concentration. 



260. Formation of insoluble phosphates. — The reten- 

 tion of soluble phosphoric acid in soils may be easily ac- 

 counted for by the fact that there are present in all soils 

 hydrated ferric oxide and hydrated silicates of alumina, 

 and frequently calcium carbonate, with which substances 

 phosphoric acid in solution would naturally form com- 

 pounds insoluble in water. Iron and aluminium phos- 

 phates are practically insoluble in water containing carbon 

 dioxide or weak organic acids such as might be present 

 in soil water. Calcium carbonate forms with a soluble 

 phosphate fertilizer some dicalcium phosphate, the 

 solubility of which in soil water is much greater than 

 the iron and aluminium phosphates. This is one of the 

 advantages of keeping a soil well supplied with lime if 

 a superphosphate fertilizer is to be used. Even the 

 tricalcic phosphate, although less soluble than the dicalcic, 

 is more readily soluble than the iron and aluminium 

 phosphates. As lime has a tendency to move downward 

 in soil, and as phosphoric acid is retained in the plowed 

 depth when added as a fertilizer, it is important that the 

 applications of lime be sufficiently frequent to keep this 

 part of the soil in a condition to form the lime phosphates. 



Cameron ^ has suggested that the absorption of phos- 

 phoric acid is probably due to the formation with lime 

 or ferric oxide of a solid solution, which might account 



^ Cameron, F. K. The Soil Solution, p. 50. Easton, Penn- 

 sylvania. 191L 



