CARBON, PHOSPHORUS AND NITROGEN IN SOILS 103 



suspended clay. They claimed that filtration through the Cham- 

 berlain-Pasteur filter, as suggested by Cameron, introduced a 

 serious error inasmuch as the filter absorbed some organic matter. 

 They proposed an evaporation method : by evaporation of the 

 ammoniacal extract to dryness re-dissolving in ammonia and filtra- 

 tion, several times, a perfectly clear solution was obtained. Deter- 

 mination of the humus in this filtrate gave very concordant results. 

 Hopkins and Pettit (68) found that in certain soils the mineral 

 composition had a tendency to be constant in the surface, sub- 

 surface, and subsoil. This was indicated by the uniform "potassium 

 content of the surface, subsurface, and subsoil and by the fact that 

 different samples of surface soil of the same type showed a wide 

 variation in the phosphorus content but that this variation largely 

 disappeared in the subsoil. The potassium exists in the soil in the 

 inorganic form, the nitrogen exists chiefly jn the organic form while 

 the phosphorus may exist in the inorganic and organic state. They 

 suggested, therefore, a method for calculating the phosphorus in 

 the organic state in the surface soil. The difference in amount of 

 nitrogen in the surface soil 'and subsoil, and the difference in the 

 amount of phosphorus in the surface soil and subsoil gave appar- 

 ently the amount of nitrogen and phosphorus associated together 

 in organic combination. By means of this ratio and the total 

 amount of nitrogen in the surface soil the total amount of organic 

 phosphorus present in the surface soil could be calculated. 



4. CARBON AND NITROGEN CONTENT OF FUNDAMENTAL ROCKS 



The fundamental rocks out of which soils have been formed 

 contain an appreciable amount of carbon and nitrogen which is in- 

 digenous to them. 



Dellese (3) discovered that mineral matter, crystalline, sedi- 

 mentary and eruptive contained carbon associated with nitrogen. 

 This mineral matter, which was formed under similar conditions 

 of temperature, pressure, etc., had a tendency to contain a constant 

 amount of carbon and nitrogen. 



The work of Lawes and Gilbert (16), Dyer (55) and Hall and 

 Miller (66) on the clays and other fundamental rock material 

 taken from various great depths indicated that an appreciable 

 amount of carbon and nitrogen was indigenous to the underlying 

 soil material. 



