CHEMISTEY OF SOILS 121 



rocks, but is only of importance when the ultimate pro- 

 perties of the soil are under consideration. 



Two methods are chiefly employed in the investigation 

 of the insoluble matter. In one, the silica is volatilised 

 in the form of silicon fluoride by treatment with hydro- 

 fluoric acid, and the bases which remain behind are dis- 

 solved in acid and examined in the usual way. In the other 

 method, the silica is converted into a soluble silicate by 

 fusion with podium carbonate. This compound is then 

 decomposed with acids ; the silica is precipitated and the 

 bases are dissolved and can be examined as before. 



The Total Acid Extract. In analyses of soils the total 

 amounts of all the mineral matters which can be dissolved 

 by concentrated acids are generally classed together and 

 may be conveniently referred to as the total acid extract, 

 without reference to the particular kind of acid that may 

 have been employed to dissolve any one or more of them. 

 The results of analyses of the fine earth of certain typica] 

 soils, showing the proportion of organic matter, insoluble 

 mineral residue and total acid extract, and the composition 

 of the last, are given in the tables on p. 122. 



It will be seen that the proportion of total acid extract 

 varies considerably in different classes of soil. In general, 

 it is greatest in soils which contain much lime and least in 

 sandy soils. In ordinary non-limey soils from 5 to 15 per 

 cent, of 4)he dry fine earth is usually soluble in concen- 

 trated acids. Oxides of iron and alumina are the principal 

 ingredients, lime and, sometimes, magnesia are present 

 in considerable quantity. The proportions of the other 

 ingredients are relatively small usually less than 1 per 

 cent., and not infrequently less than ^ per cent. The 

 large proportion of potash found in the stiff clay soil is 

 exceptional. One of the humous soils contains much 

 less potash than ;the other. Soils of that class are not 

 infrequently deficient in potash. The largest proportion 



