256 POWER OF THE SOIL TO ABSORB SALTS [chap. 



potash during fifty years' manuring being found there. 

 Some of the potash, however, sinks further and is 

 retained in the subsoil ; in the top 27 inches a large 

 proportion nearly one-quarter of the whole remains 

 in such a loose state of combination that it is soluble 

 in 1 per cent, citric acid, and so may be regarded as 

 available for the plant. 



Absorption oj Phosphoric Acid. 



The retention of soluble phosphoric acid by the soil 

 is easily intelligible on chemical grounds, for there are 

 present several substances capable of forming insoluble 

 compounds with phosphoric acid e.g., calcium car- 

 bonate, hydrated ferric oxide, and the hydrated silicates 

 of alumina which make up so much of clay. Soil has, 

 however, an adsorptive power for phosphoric acid, 

 and, as Russell has shown, the action of very dilute 

 acids upon soil can best be explained by assuming a 

 preliminary solution of the phosphoric acid followed 

 by an adsorption, the extent of which is conditioned by 

 the nature and strength of the dilute acid with which 

 the soil is in contact. 



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 1000 to 1. 



Percentage of Phosphoric Acid Absorbed (Voelcker). 



Red Loam . 

 Chalky Soil . 

 Stiff Clay . 

 Stiff Subsoil . 

 Light Sandy Soil 



