VI.] ANALYTICAL METHODS 143 



(5) Phosphoric acid and potash soluble in strong 

 hydrochloric acid. If necessary, soda, lime, 

 magnesia, oxides of iron, alumina, and 

 sulphuric acid can be determined in the same 

 solution. 



(1) About 5 grams are weighed out into a plati- 

 num dish or porcelain basin and dried in the ordinary 

 steam oven, the temperature of which is never quite 

 ioo°C. If the soil contains much organic matter, it 

 will be difficult to bring it to a constant weight, the 

 material will slowly lose water for weeks. An arbitrary 

 limit of twenty-four hours drying should be taken. 



(2) The loss on ignition should represent the organic 

 matter which is burnt to carbon dioxide and water when 

 the soil is heated in the air, but it is impossible to 

 avoid at the same time driving off some of the water 

 of constitution in the zeolites, kaolinite, and similar 

 hydrated silicates in the soil. It is difficult even to 

 obtain consistent results, because of variations in the 

 temperature and time of the operation. The best plan 

 is to heat the soil, as dried in the previous operation, 

 at as low a temperature as possible, to a barely visible 

 redness, preferably in a platinum dish, for some hours 

 with occasional stirring. 



The loss on ignition is wanted as a measure of the 

 organic matter of the soil, but we have no means of 

 estimating the varying part, great with clay soils, 

 that is played by the water of constitution. It is possible 

 to get a better measure of the organic matter by estimat- 

 ing the total carbon in the soil and assuming that the 

 organic matter of the original soil contained about 

 55 per cent, of carbon. The combustion of a soil by the 

 ordinary method for determining carbon is rather a 

 tedious process even in skilled hands ; in dealing with 

 soils it is convenient to effect the oxidation by means of 



