cnEMirAT, ACTION- IN THE SOIT.. Jj5t 



place, a mechanical analysis, as described on page 147, and 

 subjecting the fine portion, which consists entirely or in 

 great part of clay, to the action of tliese acids, the quan- 

 tity of clay may be approximately estimated. Or, by 

 melting the portion insoluble in acids with carbonate of 

 soda, or acting upon it with hydrofluoric acid, the whole 

 may be decomposed, and its elementary composition be 

 ascertained by further analysis. 



Notwithstanding an immense amount of labor has been 

 expended in studying the composition of soils, and chiefly 

 in ascertaining what and how much, acids dissolve from 

 them, we have, unfortunately, very few results in the way 

 of general principles that are of application, either to a 

 scientific or a practical purpose. In a number of special 

 cases, however, these investigations have proved exceed- 

 ingly instructive and useful. 



§5. 



REACTIONS BY WHICH THE SOLUBILITY OF THE ELEMENTS 

 OF THE SOIL IS ALTERED. SOLVENT EFFECT OF 

 VARIOUS SUBSTANCES THAT ARE COMMONLY 

 BROUGHT TO ACT UPON SOILS. THE AB- 

 SORPTIVE AND FIXING POWER OF SOILS. 



Chemical Action in the Soil. — Chemistry has proved 

 that the soil is by no means the inert thing it appears to 

 be. It is not a passive jumble of rock-dust, out of which 

 air and water extract the food of vegetation. It is not 

 simply a stage on which the plant performs the drama of 

 growth. It is, on the contrary, in itself, the theater of 

 ceaseless activities; the seat of perpetual and complicated 

 changes. 



A large share of the rocks now accessible to our study 

 at the earth's surface have once been soil, or in the condi- 

 tion of soil. Not only the immense masses of stratified 

 limestones, sandstones, slates, and shales, that cover so 



