142 SOIL CONDITIONS AND PLANT GROWTH 



humus. ^ This formation from cellulose suggests an interest- 

 ing possibility as to its origin. It has already been stated 

 that a substance resembling humus is formed on heating 

 sugar with hydrochloric acid {e.g. 250 grms. dextrose with 

 I litre 24 per cent. HCl for 12 hours). Maillard has shown 

 (189^) that the reaction is general and not confined to strong 

 acids. 



He was led by his investigations on the function of alcohol 

 in the synthesis of albuminoids to study the action of a typical 

 amino acid, glycocoll, on d-glucose. When i part of glycocoll 

 and 4 parts of glucose were added to 3 to 4 parts of water 

 and heated on the water-bath to facilitate dissolution the re- 

 action mixture rapidly changed to yellow and finally to dark 

 brown, CO2 being given off. The action was a true condensa- 

 tion and not an oxidation, for practically no oxygen was 

 absorbed. The black material had the properties of soil 

 humus : it was insoluble in water, dissolved in alkalis, but was 

 reprecipitated on adding acids, and contained 4-4 to 6 per 

 cent, nitrogen : this question is discussed at length in the later 

 paper (189^). The reaction turned out to be general, and 

 was given by all the amino acids tried (glycocoll, sarcosine, 

 alanine, valine, leucine, tyrosine, glutamic acid and also the 

 polypeptides) and all the sugars. Xylose and arabinose reacted 

 especially quickly, fructose, galactose, glucose, and mannose 

 less quickly, and lactose and maltose still less, while saccharose 

 acted only slowly. The reaction proceeds at ordinary tem- 

 peratures, but more slowly. 



Now it is certain that both amino acids and pentose com- 

 pounds are formed during the decompositions in the soil, the 

 forrner from the proteins and the latter from the celluloses, 

 and it is highly probable that the black substance is formed by 

 their interaction. A detailed study of the reaction was there- 

 fore made at Rothamsted by V. A. Beckley (12); setting out 

 from an observation by Fenton, he showed that sugars, on treat- 

 ment with acids, give rise to hydroxymethylfurfuraldehyde, 

 which readily condenses to form a substance closely re- 



1 Gortner, Soil Sci., 1917, 3, 1-8. 



