THE COMPOSITION OF THE SOIL 



141 



Schreiner and Shorey (250) have attempted a resolution 

 of the " humic acid " and of the " crenic acid " (the part not 

 precipitated by HCl) and have obtained the following sub- 

 stances from the alkaline or alcoholic extract : — 



Substances Precipitated by Acids (the so-called 

 Humic and Ulmic Acids). 



Resin acids. 



Resin esters. 



Glycerides. 



Paraffinic acid, C24H4g02, m.pt. 45°-48°, 

 probably identical with the acid 

 formed on treating paraffin with fum- 

 ing nitric acid. 



Lignocericacid, C24H48O2, m.pt. 8o°-8i°, 

 isomeric with above. 



Agroc eric acid, C21H42O3, m.pt. 72°-73°, 

 a hydroxy fatty acid. 



Agrosterol, C2gH440, m.pt. 237°. 



Phytosterol, C26H44O . HjO, m.pt. 135°. 

 Both of the cholesterol group. 



Substances not Precipitated by Acids (the so- 

 called Crenie and Apocrenic Acids). 



Dihydroxy stearic acid, Ci8H3g04, m.pt. 



98°-99°, identical with the acid formed 



on oxidising elaidic acid. 

 a-Picoline y-carboxylic acid, C7H7O2N, 



m.pt. 239°, identical with the acid 



formed on heating uvitonic acid to 



274°. 

 Xanthine, C5H4O2N4. 

 Hypoxanthine, C9H4ON4. 

 Cytosine, C4H5ON3 . H2O. 

 Histidine, CgH^OgNs. 

 Argimne, CgHj402N4. 

 A pentosan. 



None of these, however, is the black substance which is 

 the real characteristic of " humus," and until recently this had 

 eluded investigation and was vaguely described as a " melanoid 

 body ". It can be split up by solvents into the following sub- 

 stances which are no doubt further resolvable : — 



Soluble in NH4HO Insoluble in NH4HO 



I 



Soluble in acids 



(Mulder's apocrenic acid) 



Pptd. by acids 



Humus 



I 



Humin 



Insoluble in alcohol Soluble in alcohol 

 Humic acid Hymatomelanic acid^ 



I (see Schreiner and Shorey) 



Insoluble in pyridine Soluble in pyridine 



The substances are probably related : at any rate they occur 

 together. There is good evidence that soil humus is formed 

 mainly from cellulose : no great amount is produced from 

 protein. Addition to the soil of wool, silk waste, flour, and 

 lucerne meal causes no gain, but on the contrary loss of 



^ For Hoppe-Seyler's hymatomelanic and fulvic acids see Od^n, 2ig«. 



