NITROGEN IN ORGANIC MATTER 123 



percentage of nitrogen present varies greatly viz, from 

 less than 2 per cent in the humus of some humid soils 

 to more than 22 per cent in the humus of some arid 

 soils, as reported by Hilgard. His results show that 

 under arid and semi-arid conditions the humus is much 

 more rich in nitrogen than in humid regions, and he 

 attributes to this fact the large capacity of the former 

 soils to produce crops with so little organic matter. 

 His figures on this point are exhibited in the following 

 table. 



Per Cent of Nitrogen in Humus of Soil from 

 Different Regions 



Humid soils, average of sixteen samples 4.58 



Sub-irrigated arid soils, average of fifteen samples . . 8.38 

 Arid upland soils, average of forty-two samples 15.23 



The nitrogen is changed under good soil conditions 

 to forms available to plants. 



There is a similar relative increase in the proportion 

 of carbon in humus over that in the original material. 

 The coals are metamorphosed muck and peat deposits, 

 and their value for fuel lies in their carbon content. 

 Hilgard has shown by a series of analyses that there 

 is a gradual increase in the carbon content during the 

 decay process, at least up to the humus stage, which 

 is shown physically by the darkening of the material. 

 This darkening, which appears in peat and muck may 

 be the result of the separation of free carbon which, in 

 the amorphous form, is black. Its practical significance 

 in a soil way is its large effect on the color of the soil, 

 which alters its heat relations. Crops always start first 



