6o PROXIMATE CONSTITUENTS OF SOILS 



constant weight. The loss in weight is moisture. The 

 dry residue is carefully ignited in a platinum or 

 porcelain dish ; when cold, it is again weighed. The 

 loss is partly due to the liberation of combined water, 

 but mainly to the decomposition of organic matter, 

 chiefly humus. 



{b) Preparation of Humus. — About 50 grams of 

 peaty soil or well-rotted dung are treated with dilute 

 (10 per cent.) hydrochloric acid until the effervescence 

 ceases. The residue is washed several times by decan- 

 tation, transferred to a filter paper, and washed with 

 hot water until free from acid. The solid matter is 

 then removed to a beaker and shaken, or stirred, with 

 ammonia solution. If possible the soil and ammonia 

 should be left in contact for a day or two. In either 

 case the black solution of humus is poured off and 

 neutralised by the addition of strong hydrochloric acid. 



The flocculent precipitate which is formed is filtered 

 off and dried. It consists of humus. Notwithstanding 

 its method of preparation, incineration of the product 

 will generally show the presence of mineral matter 

 which has been retained. 



{c) The presence of humus in the drainage from the 

 manure-heap may be shown by the simple addition of 

 dilute HCl. A little carbonic acid is generally evolved, 

 and the humic acid is precipitated as above. 



The chemical composition of humus is indefinite, and varies 

 very considerably. It contains, as a rule, substances of the type of 

 humic and ulmic acids, in the form of salts of calcium, and these 

 must be decomposed before the humus can be separated. 



The water-retentive power of humus is greater than that of 

 clay or sand. 



88. Sand, 



(a) Ten grams of the soil are weighed out, transferred 

 to a beaker, and shaken up with a large bulk of water 



