THE ORGANIC MATTER OF THE SOIL 



147 



range from 35 to 85 per cent, according to the admixture 

 of mineral matter from extraneous sources. 



99. The humus content of soils. The humus content 

 of soils is of course lower than the organic matter therein 

 contained. It likewise varies according to climate and 

 region, not only in amount, but also in composition. The 

 following data, compiled from Hilgard, 1 illustrate this 

 point : 



THE HUMUS OF ARID AND HUMID SOILS 



It is evident that humid soils not only contain the 

 greatest amounts of organic matter, but also excel in 

 humus. The humus of the arid regions, however, is 

 richer in nitrogen, due to the peculiar decomposition going 

 on. As a consequence the nitrogen in the soil of humid 

 regions is not greatly in excess of that in the soils of drier 

 climates. 



The percentage of humus not only decreases as the 

 lower depths of soil are examined, but also changes in 

 composition, becoming poorer in nitrogen the deeper 

 the soil is penetrated. The following data on a Russian 

 river soil, quoted by Hilgard, 2 may be cited as an in- 

 stance : 



1 Hilgard, K". W. Soils, pp. 136-137. New York. 1911. 



2 Ibid., p. 139. 



