SOILS AND THEIR PROPERTIES 71 



to drain for a considerable time, the amount of water 

 remaining will vary according to the physical properties of 

 the soil, as discussed above. In the case of sands and very 

 light soils, from 5 to 10 per cent, of water will be the amount 

 commonly reached ; whilst in the case of clays and heavy 

 soils, from 30 to 50 per cent, will be held. When the conditions 

 are very varied as regards rainfall, drainage, etc., the amount 

 of water found will correspondingly vary (see p. 95). 



The ordinary figures of analysis are generally reckoned 

 on a soil which has been dried at 100 Centigrade. In some 

 cases reference is made to air-dried soils containing something 

 between 2 and 5 per cent, of water. In other cases 120 

 Centigrade is taken as the temperature for determining water. 

 To obtain a soil in complete solution only very drastic methods 

 will suffice. By ignition at a red heat the whole organic 

 matter is driven off, and by the subsequent action of hydro- 

 fluoric acid the silica is volatilized, and the remaining 

 substances go into solution. It is very rare indeed that 

 the information obtainable by solution in hydrofluoric acid 

 has any agricultural value, as neither the plant nor the soil 

 bacteria nor atmospherical agents can possibly compare 

 with hydrofluoric acid. The strongest acid commonly 

 employed in the laboratory is strong hydrochloric acid. 

 For many purposes the information obtainable from ex- 

 traction by very weak solvents is of much greater value 

 than infoimation obtainable by more drastic chemical 

 agents. Experience and convenience show that a solution 

 of I per cent, citric acid, as recommended by Dr. Bernard 

 Dyer, is one of the best of the weak solvents. It is usual 

 in laboratories to shake a mixture of the soil with I per cent, 

 citric acid by hand at intervals for three to six days, or to 

 agitate in a mechanical shaker for about twelve hours. 

 Of the ingredients usually discovered by chemical exami- 

 nation we have, among the mineral portions, the following 

 materials : 



Iron. This element occurs chiefly as ferric hydrate, 

 and partly as ferric silicates, but, under exceptional circum- 

 stances, as ferrous compounds and pyrites. All fertile 



