110 SOILS AND MANUEES 



The various substances are all closely related ; they 

 undergo constant change, and probably one is derived 

 from another by processes of oxidation and dehydration. 

 They are obtained as a whole by extracting hurnous soil 

 with ammonia solution and reprecipitating with acid. The 

 product, commonly called " humic acid," is an amorphous 

 colloidal substance, slightly soluble in water, has an acid 

 reaction and decomposes carbonates. It unites readily with 

 ammonia and alkalis forming soluble compounds called 

 "humates." The compounds with lime and other bases 

 are insoluble, and in the preparation of humic acid the 

 bases should be removed by first extracting the soil with 

 hydrochloric acid. In preparing calcium hurnate Schloe- 

 sing neutralised the dark coloured ammoniacal extract 

 till a permanent precipitate of humic acid just began to 

 form, and when this was redissolved, he added calcium 

 chloride solution and obtained a precipitate of calcium 

 humate. It is a colloidal body of great cementing power 

 (see p. 87). 



Apart from its influence on the physical properties of the 

 soil, humus owes its great importance in agriculture chiefly 

 to the nitrogen it contains. The nitrogen is not included 

 in any of the formulae ascribed to the supposed consti- 

 tuents of humus, because it has been assumed that it exists 

 in the form of a basic radicle in combination with the acid 

 compounds. However this may be, it appears to be pre- 

 sent in an insoluble form and is not, therefore, liable to 

 loss by drainage, but it is rapidly converted into soluble 

 compounds and becomes available to plants as the humus 

 decays. The proportion of nitrogen, like that of the other 

 constituents of humus, is variable. Mulder found from 

 2*5 to 4'0 per cent., and Kostytcheff from 4*0 to 6*55 per 

 cent, in different samples. Owing to the elimination of 

 carbon and hydrogen by oxidation there is a tendency for 

 the proportion of nitrogen to increase as the humus decays 



