46 THE MECHANICAL ANALYSIS OF SOILS [chap. 



Gilbert for the ratio that exists between the propor- 

 tions of carbon and nitrogen in various soils : — 



Hilgard and Jaffa also found that the humus of soils 

 in an arid climate, where the deficiency of rainfall causes 

 the soil to be very open, contains a higher proportion of 

 nitrogen than is found in the humus of damper soils : — 



The following table (p. 47) gives the results of the 

 determination of carbon, nitrogen, humus, and the 

 percentage of nitrogen in the humus, in a selection of 

 extremely rich virgin soils obtained from different 

 parts of the world ; the Canadian, Russian, and Monte 

 Video soils were very similar uniform fine-grained grey 

 or black soils found on the great plains. 



These results would seem to indicate that the most 

 valuable humus, i.e. that which will decay rapidly 

 and yield nitrogen compounds available as food for 

 plants, is that possessing a high ratio of carbon to 

 nitrogen. 



