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ABSORPTIVE POWER OF THE SOIL. 357 
retained rather than altered to ammonia or some other 
compound. 
The fixation of acids in the 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 chlorides, nitrates, sulphates, silicates, 
phosphates and carbonates of these oxides. Whether such 
compounds can be actually produced 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 free 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 disappeared 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 
