144 SOILS: PROPERTIES AND MANAGEMENT 



due to the resistance to oxidation 1 by the carbonized 

 matter, already discussed. This material, while it suc- 

 cumbs to ignition, resists the action of the sulfuric and 

 chromic acids to a very large degree. 



97. The estimation of soil humus. — The common 

 method of humus estimation is that proposed by Grand- 

 eau. 2 The sample of soil is first washed with acid in order 

 to remove all bases. It is next treated with ammonia, 

 which will then dissolve out the humous materials. By 

 catching this percolate, evaporating it to dryness, and 

 weighing it, the percentage of humus may be calculated. 

 The dark humous extract obtained with the ammonia 

 is called the Matiere Noire. 



This method has undergone several modifications, 3 of 

 which that of Ililgard 4 and that of Houston and McBride 5 

 seem the most promising. The method of the latter 

 chemists has been adopted by the Association of Official 

 Agricultural Chemists and is considered as the official 

 method. In the procedure an attempt is made to keep 

 the concentration of the ammonia in contact with the 

 soil constant during the extraction. Consequently the 

 sample, after treatment with the acid, is washed into a 



1 Schreiner, O., and Brown, B. E. Occurrence and Nature 

 of Carbonized Material in Soils. U. S. D. A., Bur. Soils, 

 Bui. 90, pp. 19-21. 1912. 



2 Grandeau, L. Traiti d'Analyse de Matieres agricoles. I, 

 p. 151. 1897. 



3 A comparison of the various methods is found as follows : 

 Alway, F. J., and others. The Determination of Humus. 

 Nebr. Agr. Exp. Sta., Bui. 115. June, 1910. 



4 Hilgard, E. W. Humus Determination in Soils. U. S. 

 D. A., Div. Chem., Bui. 38 (edited by H. W. Wiley), p. 80. 

 1893. 



5 Wiley, H. W. Official and Provisional Methods of Analy- 

 sis. U. S. D. A., Bur. Chem., Bui. 107, p. 19. 1908. 



