Nitrogen Fixation 37 
amount of silicic acid set free upon digestion with hydrochloric 
acid. The carbonic acid may be considered as existing wholly as 
calcic carbonate which is essentially correct, for the calcic carbonate 
can be detected in most of the soils by simple physical examination. 
The humus in our soils is usually low in percentage and though 
apt to be richer in nitrogen than the humus of states with a higher 
rainfall it is not always so. The total nitrogen content of our soils 
as indicated by a fairly large number of analyses is not far from 
0.10 per cent. It follows that a great many of them are below this 
amount. The four samples given illustrate this and show that 
there is no excessively large quantities of nitrogen present in the soil. 
The nitrogen determinations were made by the plain Kjeldahl, as 
it was not supposed that the nitrates were present in sufficient quan¬ 
tities to influence the results, assuming that this is correct it would 
appear that about one half the nitrogen in number 697 was present 
in the form of nitrates for we found 0.037 P er cent of nitrogen 
calculated on the air dried soil in the aqueous extract as nitrates. 
The amount of nitrates present in the soil at a given time seems 
never to have been made the subject of study, probably because they 
are very small. The Rothamsted experiments lead to the conclusion 
that in good agricultural land cultivated as bare fallow about 80 
pounds of nitrogen were transformed into nitrates in 14 or 15 
months between the removal of the preceding crop and the taking 
of the samples. This includes the nitrates carried out of the land 
by drainage. The source of these nitrates is not considered. They 
may have been and probably were largely formed partly by nitrify¬ 
ing processes, partly by fixation. Storer further states “In three 
fields at Rothamsted 56.5, 58.8 and 59.9 pounds of nitrogen in the 
form of nitrates were found in September and October taking the 
soil to a depth of 27 inches. In one of these fields 49 pounds of nitric 
nitrogen occurred in the uppermost 18 inches. This result was due 
to the richness of the soil in nitrifiable matter. The other two fields 
in less favorable agricultural conditions contained only 33.7 and 36.3 
pounds of nitrate nitrogen in the uppermost 18 inches of soil.” Lip- 
man states that “The determinations of nitrates, at Rothamsted, 
in the drainage waters of land that had been kept fallow, showed 
an annual removal of 40.2 pounds of nitrate nitrogen per acre.” 
The maximum of nitric nitrogen here given, 59.9 pounds, gives 
only 359.4 pounds of nitrate of soda per acre taken to the depth 
of 27 inches; while we found in number 697, 9,040 pounds in an 
acre foot after the rich surface portion had been removed. This 
surface portion contained in the uppermost four inches of soil 
22,747 pounds of sodic nitrate. It is understood that sodic nitrate 
is used simply as a convenient form of expression and is not in¬ 
tended as a definite statement that there were no magnesic or calcic 
