viii.] PHOSPHORIC ACID 257 



In an ordinary soil containing a sufficiency of 

 calcium carbonate, the application of soluble phosphoric 

 acid like superphosphate will chiefly result in the 

 precipitation of di-calcium or " reverted " phosphate, 

 wherever the solution meets with a particle of calcium 

 carbonate. This di-calcium phosphate is a compound 

 easily soluble in weak organic acids or in water con- 

 taining carbonic acid : hence the great value of applica- 

 tions of superphosphate on soils rich in lime, for thus a 

 readily available phosphate is very quickly disseminated 

 throughout the ground in a state of fine division. But 

 on soils poor in calcium carbonate the precipitation will 

 be chiefly effected by the hydrated iron and aluminium 

 compounds, and the resulting phosphates are practically 

 insoluble in water containing carbonic acid, and but 

 little in saline solutions or in weak organic acids. 

 Hence applications of superphosphate to such soils 

 become much less available to the crop, and should be 

 preceded by a thorough liming of the land. Even a 

 subsequent liming on soils containing phosphates of 

 iron or alumina will help to bring them into a more 

 available form, because a double decomposition result- 

 ing in calcium phosphate and aluminium or ferric 

 hydrate will proceed to an extent dependent on the 

 mass of lime present in the medium. 



Further evidence of the precipitation of phosphoric 

 acid within the soil is afforded by Dyer's examination 

 of the Rothamsted wheat soils at various depths, after 

 fifty years' continuous manuring with and without super- 

 phosphate. By comparing the amount of phosphoric 

 acid contained in the soil of the unmanured plot 

 with that contained in the soils of the plots receiv- 

 ing superphosphate every year, and knowing also the 

 amount removed by the successive crops in each case, 

 it is possible to calculate the surplus that should 



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