THE COMPOSITION OF THE SOIL 143 



sembling humus. He also found indications of hydroxy- 

 methylfurfuraldehyde in a dunged soil and in rotting straw in 

 which humus was being produced. He suggests, therefore, 

 that the formation of humus in the soil proceeds in two 

 stages : — 



Carbohydrate (Cellulose, etc.) ( + Amino acid) -^ 



Hydroxymethylfurfuraldehyde. 

 Hydroxymethylfurfuraldehyde + amino acid -» humus + 



furfural + CO2 by condensation. 



Eller and Koch,^ on the other hand, suppose that humus 

 is formed by the oxidation of quinones which arise by the 

 elimination of water from hexoses ; — 



CgH^.O, = 4H2O + C,Hp, 



Beijerinck (17) has already found quinone among the products 

 of certain soil organisms working in culture solution. 



As to the constitution of humus little is known, Sven 

 Oden of Upsala {2\%d) has adduced evidence from the con- 

 ductivity of the solution in ammonia that the alcohol insoluble 

 portion is a true acid, as was supposed by the older chemists 

 and by Tacke (278). Ehrenberg and Bahr (93^) also agree 

 with this view : they show that it neutralises sodium hydroxide, 

 the point of neutralisation being sharply indicated by con- 

 ductivity measurements. On dilution the sodium compound 

 behaves as a salt or a tri- or tetra-basic acid. Further evi- 

 dence of true acidic nature is afforded by the fact that the 

 alkaline solution behaves as a true solution, and shows none 

 of the properties of colloids (Oden, 2 1 2)b). There is, however, 

 another view (see p. iii) that the acidic properties are the 

 result of the colloidal condition and not of a true acid con- 

 stitution, but it is improbable that this is a complete explana- 

 tion of the phenomena. 



For a long time the humus soluble in alkalis was supposed 

 to play a great part in determining fertility: Grandeau (li2a) 

 and Hilgard (133) especially considered it to be the most 



^ Wilhelm Eller and Kate Koch, Ber., 1920, 53 B, 1469- 1476. 



