HUMUS lllMOfiKN. 



241 



connection it may be pointed out tliat Ililgard insists that calcium 

 carbonate is the factor which determines the transformation of 

 plant residues into mild humus. 



Be this as it may. one fact seems to stand out very clearly, i.e., 

 that there are two chief types of humus, viz. : — 



(a) Acid Humus. 



(b) Neutral or Alkaline Flumus. 



Of the two types the latter is the valuable and desirable one. So 

 much for the classification of humus types. Although much has 

 been accomplished in the resolution of humus into its various 

 constituents, there remains still more to accomplish. A good deal 

 has been done in the way of grouping these constituents, and in 

 elucidating the connection between the natural and artificial 

 varieties, viz.. those found in the soil and those made from sugars 

 by boiling with acids. 



(9) Schreiner and Shorey grou]) the constituents of humus 

 as follows : 



When the soil is decomposed by treatment with acid and then 

 extracted with alkali, a part of the humus (the tnatiere noire dc 

 Grandeau) goes into solution and a part remains undissolved. 

 The insoluble portion is called the Humin Group. So far. it has 

 defied all efforts to analyse it. If the alkaline solution be acidi- 

 fied a precipitate is formed, which, however, does not contain all 

 the matter that the alkali dissolved from the soil ; a part remains 

 in solution. To this portion is assigned the designation Crenic 

 Acid Group. The precipitate is the Humic Acid Group. This 

 is still further subdivided into soluble and insoluble humic acids, 

 both of which are groups of compounds. The differentiation be- 

 tween the two is dependent on their different behaviour towards 

 alcohol. The compounds grouped as Insoluble Humic Acid com- 

 prise that poi"tion of the ])recipitate obtained on acidifying the 

 alkaline soil extract which is insoluble in alcohol. 



The whole grou})ing is concisely illustrated hereunder : 



Humus, extract with alkali 



Alkaline solution, acidify Insoluble residue of 



HUMIN, which 

 only dissoKes 



I I on fusion with 



Remaining in Precipitate, wash, alkalies, 



solution boil with alcohol 



CRENIC ACID. ! I 



Remaining in- Going into 



soluble solution 



I I 



INSOLUBLE SOLUBLE 



HUMIC ACID. HUMIC ACID. 



