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SIR J. B. LA WES AND PROFESSOR J. H. GILBERT ON THE 
growing plant', very strikingly illustrated and confirmed the result of the exhaustion 
of the nitric acid of the subsoil hy the strong, deep-rooting, and high nitrogen- 
yielding Leguminosse. Still, the figures did not justify the conclusion that the whole 
of the large amount of nitrogen taken up by the Meclicago crops could have had its 
source in nitric acid. It was obvious that much nitrification takes place near the 
surface; hut as the surface soil became even somewhat richer in nitrogen, it was 
clear that it had not been the primary source of the whole of the nitrogen taken up 
by the plants. The source of much of it must have been either the atmosphere, or 
the subsoil; and if the subsoil, and yet not wholly as nitric acid, the question 
arises, in what other form of combination 1 
In another experiment, where one leguminous crop—beans—had been sown for 
many years in succession, but had frequently yielded very small crops, and sometimes 
failed, and over the whole period had given an average of little more than 30 lbs. of 
nitrogen per acre per annum, the land was then left fallow for several years, after 
which, in 1883, barley and clover were sown. In that year, in 1884, and in 1885, 
about 300 lbs. of nitrogen were removed per acre, chiefly in the clover crops. This 
result was obtained—where another leguminous crop had to a great extent failed, 
where the surface soil had become very poor in total nitrogen, where there existed a 
very small amount of ready formed nitric acid to a considerable depth, and where the 
surface was unusually poor in nitrogenous crop residue for nitrification. 
Further, not only had this large amount of nitrogen been removed in the clover 
crops, but the surface soil became determinably richer in nitrogen. Here again, then, 
the primary source of the nitrogen of the crop could not have been the surface soil 
itself. It must have been either the atmosphere, or the subsoil; and assuming it to 
be the subsoil, the question arises whether it was taken up as nitric acid, as ammonia, 
or as organic nitrogen ? 
The various results adduced could leave no doubt that nitric acid was an important 
source of the nitrogen of the lieguminosee. Indeed, existing evidence relating to 
nitric acid carries us quantitatively further than any other line of explanation. 
But it is admittedly inadequate to account for the amounts of nitrogen taken up, 
either by the Medicago saliva on the clover-exhausted land, or by the clover on the 
bean-exhausted land. 
Direct experiments were made to determine whether the nitrogen of the Rotham- 
sted raw clay subsoils, from which it was assumed much nitrogen had been derived in 
some way, was susceptible of nitrification, provided the nitrifying organisms, and 
other necessary conditions, were present. It was found that the nitrogen of such 
subsoils, containing only between 0'04 and 0'05 per cent, of nitrogen, and not more 
than six or eight parts of carbon to one of nitrogen, was susceptible of nitrification. 
It was also found that nitrification was more active in leguminous, than in gramineous 
crop subsoils. 
Although it was clear that the nitrogen of raw clay subsoils, which constitutes 
