144 THE CHEMICAL ANALYSIS OF SOILS [chap. 



a mixture of sulphuric and chromic acids, taking care to 

 interpose a tube of heated copper oxide between the 

 flask containing the soil and acids and the apparatus 

 used for absorbing the carbon dioxide, in order to 

 complete the oxidation of some of the products formed. 

 This process can be made to follow the determination of 

 the carbon dioxide evolved by the action of acid alone, 

 the same apparatus and the same portion of soil serving 

 for both. 



(3) The nitrogen in 10 to 20 grams of the ground 

 " fine earth " is estimated by Kjeldahl's process. Though 

 there is some nitrate present in the soil, no special 

 precaution need be taken on its account, the proportion 

 it bears to the total nitrogen is so small as to be 

 negligible. 



(4) The earthy carbonates of the soil are estimated 

 from the quantity of carbon dioxide, which is evolved 

 on treating the ground "fine earth" with an acid. 

 When the proportion of calcium carbonate is high the 

 determination can be made by the usual gravimetric 

 methods. Scheibler's apparatus for measuring the 

 volume of carbon dioxide evolved is suitable when the 

 proportion of calcium carbonate does not fall below 

 1 per cent. ; below that point some other method must 

 be employed, because all the carbon dioxide evolved goes 

 into solution in the reacting acid. The most exact and 

 convenient method for determining calcium carbonate, 

 especially when the quantity involved is very small, 

 consists in absorbing the carbon dioxide evolved by 

 dilute caustic soda in a Reiset tower, and estimating the 

 carbon dioxide by titrating the alkali first with phenol- 

 phthalein and then with methyl orange as an indicator 

 (see Amos. Jour. Agric. Sa\, 1, 1905, 322). Certain acid 

 soils rich in humus contain other organic substances 

 which yield carbon dioxide on boiling with dilute acid, 



