THE ORGANIC MATTER OP THE SOIL 115 



be determined on a separate sample and deducted from the 

 figure obtained by the combustion above described. This will 

 give the organic carbon of the soil in terms of carbon dioxide. 

 The percentage of organic carbon may now be calculated as 

 well as the approximate amount of organic matter (C X 1.724 

 = organic matter or COg X .471 = organic matter.)^ 



61. Determination of soil humus. — Humus ^ is a term ap- 

 plied to that portion of the organic matter which can be re- 

 moved with ammonium hydroxide after the soil has been 

 treated with hydrochloric acid and washed free thereof. The 

 common method of humus estimation is that proposed by 

 Grandeau.^ The sample of soil is first w^ashed with acid in 

 order to remove the bases in combination with the organic mat- 

 ter. It is next treated with ammonia, which will then dissolve 

 out the humous materials. The method is based on the fact 

 that when a soil is lacking in active basic material, certain 

 parts of the organic matter are soluble in an alkali. The dark 

 humous extract obtained with the ammonia is called Matiere 

 Noire and is supposed to be the most active part of the soil 

 organic matter. 



This method has undergone several modifications ^ of which 



* "Wiley presents the following comparisons of the three methods dis- 

 cussed above: 



Soil 



Ignition 



Combustion 

 (e X 1.724) 



Chromic acid 

 (e X 1.724) 



Old pasture 



9.27 

 7.07 

 5.95 



6.12 

 4.16 

 2.44 



4.84 



ISTew pasture 



3.32 



Arable soil 



2.03 







Wiley, H. W., I'nnciples and PracUces of Agncultiiral Analysis, Vol. 

 1, pp. 352-354, Easton, Pa., 1906. 



*The term '^humus'' is used in a number of different ways. Conti- 

 nental Europeans make it synonymous with organic matter. In some cases 

 it is used to indicate all of the partially decayed material of the soil. The 

 restricted meaning employed m this text is less confusing as it coincides 

 with the chemical interpretation. Grandeau believed the organic matter 

 thus dissolved was a determining factor in soil fertility. 



^G-randeau, L,, Traiii d' Analyse de MaUeres Agneoles; I, p. 151, 1897. 



*A comparison of the various methods is found as follows: Alway, 



