TRANSFORMATION OF ORGANIC MATTER 697 



Portion soluble in alkali — Ulmic acid — C 40 H 2 8Oi 



Substances formed , 



, . . .. (modern iormula) 



under aerobic condi- < _, , . . , , , 



ditions : 



Substances formed 

 under anaerobic con- 

 ditions: 



Portion insoluble in alkali — Ulmin — C40H32O14 

 (modern formula) 



Portion soluble in alkali — Humic acid — CtoH 3l X)i5 



(modern formula) 



Portion insoluble in alkali — Humin — C^H^Oio 



(modern formula) 



The ulmic bodies (in the brown humus) were considered as the first products of 

 decomposition of the plant material in the soil and as occurring with the absorp- 

 tion of oxygen, evolution of C0 2 , and elimination of water. The humic bodies 

 (in the black humus) were considered as the second stage in the decomposition 

 of the plant material. The formation of apocrenic acid (C24H12O12) and crenic 

 acid (C04H24O16) were considered as the third stage in the decomposition of 

 organic matter in the soil. These acids were not precipitated on acidification 

 of the alkaline solution. Crenic acid was a colorless substance and, on exposure 

 to the air, was oxidized to apocrenic acid; the former could be obtained from the 

 latter by reduction with zinc and HC1. The nitrogen was considered to be present 

 in the soil humus in the form of ammonium or related compounds, since most of 

 it was given off as ammonia on boiling with potassium hydroxide. All these 

 theories and formulae were based of course on mere speculation and not on ex- 

 perimental evidence. 



On the other hand, the physical chemist considered that the organic 

 matter of the soil is not present there in the form of "humic acids" and 

 "humates," or definite chemical compounds of calcium and magnesium, 

 but in the form of complex colloidal systems which form adsorption 

 compounds with the various cations present in the soil. The colloidal 

 nature of "humus" was first recognized by Van Bemmelen 76 in connec- 

 tion with his studies on the absorptive properties of soils. "Humic 

 acid" is a hydrosol and can be flocculated by concentrated electrolytes, 

 especially polyvalent cations (calcium). Similar to silicic acid, it is 

 electronegative, but the charge can be reversed by certain electrolytes. 

 Freezing causes the flocculation of "humus" in soils. "Humus" is 

 capable of absorbing large quantities of water and swelling up. It also 

 absorbs nutrient minerals in the soil, particularly bases. The exact 



76 Mulder, J. J. Die Chemie der Ackerkrume., 1: 308. 1863; Hoppe-Seyler, 

 F. Uber Huminsubstanzen, ihre Entstehung und ihre Eigenschaften. Ztschr. 

 physiol. Chem., 13: 66-121. 1889; Hermann, R. Untersuchungen fiber den 

 Moder. Jour, prakt. Chem., 22: 65-81; 23: 375-386; 25: 189-206. 1841-1842; Van 

 Bemmelen, J. M. Die Absorptionsverbindungen und das Absorptionsvermogen 

 der Ackererde. Landw. Versuchst., 35: 69-136. 1888; Die Absorption, 81-144. 

 Dresden. 1910. 



