Feb. 14, 1916 
Nitrogen Content of Humus of Arid Soils 
911 
was similarly treated with a 4 per cent potassium-hydroxid or a 3 per cent 
sodium-hydroxid solution. The ammonia solution was used for the 
determination of the humus, while in the other the humus nitrogen was 
determined by the Kjeldahl method. On the assumption that the same 
compounds had been dissolved by the two solvents, the percentage of 
nitrogen in the humus was calculated. 
While Hilgard’s conclusions were based upon determinations in which 
the humus was extracted with an alkaline hydroxid solution, he later sug¬ 
gested as an alternative the use of the ammonia solution (8, p. 22), this 
being concentrated and then mixed with magnesia and boiled before 
being subjected to the Kjeldahl determination. 
The correctness of the assumption that the ammonia solution dissolves 
the same compounds or the same proportions of the total nitrogen as the 
alkaline hydroxids is open to serious question. A mere determination of 
the nitrogen removed by the two solvents does not suffice to decide the 
question. The ammonia is likely to combine with some of the dissolved 
organic matter of the soil, with the result that after the concentration 
of the extract, preliminary to the Kjeldahl digestion, there may still be 
present some nitrogen derived from the ammonia in addition to that 
extracted from the soil. The attempt to eliminate any such combined 
nitrogen by digestion with magnesia previous to the Kjeldahl determina¬ 
tion is unsatisfactory, as the magnesia may decompose some of the nitro¬ 
gen compounds extracted from the soil with the elimination of ammonia. 
A determination of the organic carbon in both solvents should be made, 
and if this is not the same the nitrogen in the alkaline hydroxic^ solution 
is not to be regarded as that corresponding to the whole of the organic 
matter dissolved by the ammonia. 
EXPERIMENTAL WORK 
We have confirmed Hilgard and Jaffa's (10) observation that after 
prolonged extraction of a soil with either ammonia or alkaline hydroxid 
solution the other fails to extract any appreciable amount of black 
material. Using 10-gm. portions of both a semiarid and a humid soil, 
we treated with a 4 per cent ammonia solution until the washings became 
colorless, placed the residues together with 500 c. c. of alkaline hydroxid 
solution in stoppered bottles, shook these at frequent intervals for eight 
hours, and then allowed them to stand overnight. In the case of potas¬ 
sium hydroxid, we tried concentrations of 64, 32, 16, 8, 4, 2 per cent and 
of sodium hydroxid of 36, 18, 9, 4.5, 2.25 per cent. In all cases the 
amount of coloring matter extracted was so small that the humus could 
not be satisfactorily determined even by the delicate photometric 
method (2). Accordingly, it seems safe to assume that the ammonia 
solution removes the dark coloring matter as completely as the alkaline 
hydroxids. However, there appears no reason for assuming that a 
definite relation exists between the quantity of this pigment and the 
