ABSORPnVE POWER OF THE SOIL. 357 



retained rather than altered to ammonia or some other 

 compound. 



The fixation of acids in tlie soil is unquestionably, for 

 the most part, a chemical process, and is due to the for- 

 mation of comparatively insoluble compounds. 



Hydrated oxide of iron and hydrated alumina are 

 capable of forming highly insoluble compounds with all 

 the mineral acids of the soil. The chemist has long been 

 familiar with basic clilorides, nitrates, sulphates, silicates, 

 phosphates and carbonates of these oxides. Whether such 

 compounds can be actually jjroduced in the soil is, how- 

 ever, to some extent, an open question, especially as re- 

 gards chlorine, nitric and sulphuric acids. Their forma- 

 tion must also greatly depend upon what other substances 

 are present. Thus, a soil rich in these hydrated oxides, 

 and containing lime and the other bases in minuter quan- 

 tity (except as firmly combined in form of silicates,) would 

 not unlikely fix free nitric acid or iree sulphuric acid as 

 well as the chlorine of free hydrochloric acid. When the 

 acids are presented in the form of salts, however, as is 

 usually the case, the oxides in question have no power to 

 displace them from these combinations. The acids, can- 

 not, therefore, be converted into basic aluminous or iron 

 salts unless they are first set free — unless the bases to 

 which they were previously combined are first mastered 

 by some separate agent. In the instance before referred 

 to where nitric acid disappeaied from a soil, Knop sup- 

 poses that a basic nitrate of iron may have been formed, 

 the soil employed being, in fact, highly ferruginous. 

 The hydrated oxides of iron and alumina do, however, 

 form insoluble compounds with phosphoric acid^ and may 

 even remove this acid from its soluble combinations with 

 lime, as Thenard has shown, or even, perhaps, from its 

 compounds with alkalies. 



Phosphoric acid is fixed by the soil in various ways. 

 When a phosphate of potash, for example, is put in 



