68 CHEMICAL PROPERTIES OF SOIL 



the humus, silicates, or calcium carbonate, and partly physical, as 

 in the absorption of nitrogenous organic compounds, colouring 

 matters, etc. The latter phenomenon is termed " adsorption." 



46. Absorption of Colouring Matter and Free Ammonia. 



A little cane sugar is heated in a porcelain dish until 

 it darkens and swells up. The mass is allowed to cool, 

 and then extracted with water to which has been added 

 a drop or two of ammonia solution. The brown- 

 coloured liquid is then filtered through a large funnel 

 packed with a stiff soil, and the purification of the 

 filtrate noted with regard to the diminution of colour 

 and smell. The experiment should be repeated with 

 sand, or a very sandy soil, which will be found much 

 less effectual. 



47. Absorption of Salts {Qualitative). 



About lOO grams of a clay soil are mixed in a 

 flask with 50 c.c. of a solution containing o-i per cent, 

 each of ammonium sulphate, potassium chloride, sodium 

 phosphate, and sodium nitrate. After allowing to 

 stand, with occasional vigorous shaking, for half an 

 hour, the liquid is filtered, and tested for the various 

 salts as described below. At the same time similar 

 comparative tests are carried out with the original 

 solution, care being taken to use the same volumes of 

 solution and of reagents in both cases. 



{a) Combined Ammonia. — One c.c. of each solution 

 is mixed with a, little oxalic acid solution, made up to 

 100 C.C., and the precipitated calcium oxalate allowed 

 to settle. By means of a pipette, 50 c.c. of the 

 clear liquid is transferred to a Nessler cylinder, mixed 

 with 2 c.c. of Nessler's reagent, and allowed to stand 

 for five minutes. The depth of colour produced in each 

 case is to be noted. 



