158 Estimation of Calcium Carbonate in Soils 



alone, it seemed impossible in such cases to obtain an accurate estimation 

 of the carbonate by any method in which the soil was subjected to the 

 decomposing effect of water boiling under atmospheric pressure. 



Extraction with ammonium sulphate in the cold was next tried. 

 The soil was shaken for twelve or more hours with a strong solution 

 of ammonium sulphate, and allowed to stand till the supernatant liquid 

 was quite clear. An aliquot portion was then pipetted off by means of 

 a filter pump (to avoid disturbing the fine sediment at the bottom of the 

 extraction flask), and the carbon dioxide estimated by boiling in Amos' 

 apparatus, a little sulphuric acid being added to prevent ammonia from 

 reaching the absorbing Reiset tower. While negative results were got 

 for carbonate in the acid soils tested, the normal soils always showed 

 carbonate though in quantities below those estimated by direct treatment 

 with acid. The carbonate could always be determined with a considerable 

 degree of accuracy by the following procedure. First of all, the carbon 

 dioxide was estimated by Amos' method. The same amoimt of soil was 

 then boiled with dilute hydrochloric acid for a similar period of time 

 under like conditions after standing in a vacuum as described in the 

 first experiment to verify the decomposition of organic matter. The 

 figure for the carbon dioxide evolved from carbonate was found by sub- 

 tracting the amount of carbon dioxide evolved in the latter estimation 

 from the total found in the former. The method is not free from 

 objection owing to the difficulty of maintaining the experimental con- 

 ditions exactly similar, but can be relied ou as giving very satisfactory 

 results. In normal alkaline soils containing 1 — 2 "/o carbonate of lime, 

 the amount of carbon dioxide evolved on boiling with pure water was 

 on the average 44 milligrams of carbon dioxide per 100 grams soil 

 which corresponds to 01 "/o carbonate of lime. As the ammonium 

 chloride method gave results that were much too low in comparison 

 with those obtained in the manner described, it was abandoned as 

 unreliable. 



Extraction with water supersaturated with carbon dioxide also failed 

 to give satisfactory results. The excess of carbon dioxide was boiled 

 off and acid added to decompose the precipitated carbonate, but the 

 results obtained were very erratic. 



Finally, a distillation with very dilute acid at reduced pressures 

 was tried and adopted as giving results which were very satisfactory 

 compared with those obtained by distilling the soil under atmospheric 

 pressure. Transvaal III, which, as the ammonium sulphate extraction 

 showed, contained no carbonate, yielded when boiled with water alone 



