CH.[iii] Soluble Humus 39 



precipitate. On drying, this shrinks very much to little 

 lumps almost black in colour which readily burn and 

 leave behind a Httle red ash. Its composition varies con- 

 siderably, but after it is thoroughly dried in a steam 

 oven it usually contains about 50-57 per cent, of carbon, 

 35 per cent, of oxygen, and 3-8 per cent, of nitrogen. 



The soil which has thus been treated still contains 

 more of the soluble material, and several successive ex- 

 tractions have to be made with alkah before anything 

 like exhaustion appears ; even then soluble material still 

 continues to come out although the solution is no longer 

 dark coloured. But even after numerous extractions a 

 considerable amount of the organic matter often about 

 one-haK of the original quantity remains in the soil. 

 Some of this is capable of being transformed into soluble 

 substances by heat : thus if a fresh portion of the soil is 

 heated by steam at 100 C. for an hour and then sub- 

 mitted to the extraction processes already described, a 

 considerably larger amount of material dissolves out in 

 the alkah than before. 



The chemical nature of these various substances is 

 under investigation at the Bureau of Soils, Washington, 

 and at Rothamsted. From the Rothamsted experi- 

 ments it does not appear that the "soluble humus" is 

 of particular value in soil fertility though humus as 

 a whole is highly important. The physical effects of 

 "humus" will be taken in detail in Chapter VI; they 

 faU under three headings : 



1. The organic matter imparts a black colour to the 

 soil unless there happens to be a good deal of chalk 

 present, when the white colour persists. It is a weU- 

 known physical law that a black substance absorbs 

 more heat than a white one placed under similar con- 



