220 POWER OF THE SOIL TO ABSORB SALTS [chap. 



have either soil, clay, or peat which have been pre- 

 viously washed with hydrochloric acid. 



The following table shows the percentages of the 

 total phosphoric acid supplied, which were removed 

 from solution by various soils after remaining in con- 

 tact for the specified times, the ratio between soil and 

 phosphoric acid being about iooo to E. 



Percentage of Phosphoric Acid Absorbed (Voelcker). 



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 



