THE COMPOSITION OF THE SOIL 



139 



In order to understand much of the chemical work that 

 has been done, it is necessary to remember that the older 

 chemists regarded humus as a single definite substance, or as 

 a mixture of two or three definite substances. The favourite 

 view was to consider neutral humus as the calcium salt of 

 ** humic acid," which could be extracted from the soil by dilute 

 alkalis after preliminary treatment with hydrochloric acid. On 

 acidifying this alkaline extract the " humic acid " came down 

 as a brown colloidal precipitate. Acid humus was the actual 

 humic acid itself. It was further supposed that humic acid 

 could be synthesised by boiling sugar with hydrochloric acid, 

 on the singularly inadequate ground that the product thus 

 obtained is also a brown colloid, soluble in alkalis and pre- 

 cipitated by acids. Numerous analyses have been made both 

 of the natural and the synthetic " humic acid," some of which 

 are given in Table XL.^ 



Robertson, Irvine, and Dobson give the formula CggHggOi^ 

 to humus from sugar and C24H24O11N to the soil humus ob- 

 tained through the potassium salt ; the latter may be related 

 to the acid prepared from Dopplerite by Mayer,^ containing 



^ Many partial analyses have been made. Cameron and Breazeale (65a) in 

 nineteen samples obtained percentages of carbon varying from 33*3 to 50T 

 wrhilst Hilgard (133^) found the nitrogen content to be : — 



C. B. Lipman, however, is convinced that there is an error here : he has not 

 found more than 7 or 8 per cent, of N in the humus from arid soils : this is no 

 more than occurs in the humus from humid soils {Soil Science, 1916, i, 285-290). 

 He also finds no difference in general nitrifying power {Journ. Ag. Research, 1916, 

 7, 47). Westermann (302) has analysed humus from the Danish moors, and Gully 

 (116) has studied humus from South Bavarian moors. Many of the older analyses 

 have been collected by Wollny (318). 



'Landw. Ver sucks, 1883, 29, 313, 



