ON THE GROWTH OF LEGUMINOUS CROPS. 39 



Among the 69 experiments made in this new series, there was no 

 failure to produce nitrification by samples down to 2 feet ; there was 

 only one failure out of 11 trials down to 3 feet ; but below 3 feet, the 

 failures were more numerous. Taken at 6 feet, about half the samples 

 induced nitrification. The order of priority of nitrification diminished 

 from the upper to the lower depths ; indicating more sparse occurrence, 

 or more feeble power of development and action. There was, moreover, 

 notably more active nitrification with the Leguminous than with the 

 Gramineous crop sub-soils. 



It is, then, established, that the nitrogenous matters of raw clay 

 sub-soils are susceptible of nitrification, if the organisms, with the 

 other necessary conditions, are present. It is further indicated, not 

 only that the action is more marked under the influence of Leguminous 

 than of Gramineous growth and crop-residue, but that the organisms 

 become distributed to a considerable depth, even in raw clay sub-soils, 

 especially where deep rooted and free growing Leguminosge have 

 developed. 



The next question is, how far, in a quantitative sense, do the 

 results aid us in explaining the source of the large amounts of nitrogen 

 taken up by some Leguminous crops 1 



In the case of three Leguminous crop sub-soils there was, over the 

 total period, only about 1 part of nitrogen nitrified per million of soil ; 

 and as the sub-soil, to the depth experimented on, would weigh about 

 30 million Ibs. per acre, the amount of nitrification supposed would 

 represent only about 30 Ibs. of nitric-nitrogen per acre. Obviously the 

 conditions of nitrification in which the samples are exposed in the 

 Laboratory are very different from those of the sub-soil in situ. Thus, 

 whilst in the case of the samples in the Laboratory, the conditions as 

 to temperature, and, perhaps of aeration, would be the more favourable, 

 the successive extractions by water under pressure, would be liable to 

 remove, not only the mineral matters essential for the development of 

 the organisms, and for the production of nitric acid, but the organisms 

 themselves ; whereas, in the case of the natural sub-soil, the tendency 

 would be to multiplication. 



Compared with the small amount of nitrification of the nitrogen of 

 the raw clay sub-soils shown in the foregoing experiments, some results 

 obtained by Mr. Warington, in experiments in which he mixed raw clay 

 sub-soil with an equal weight of coarsely powdered flint, seeded the 

 mixture with rich garden-soil, moistened it, and placed it in a vessel 

 allowing of free access of washed air, show, in one case 12 - 9 and in 

 the other 1 1 *8 parts of nitrogen nitrified per million of sub-soil ; and, 

 when mineral plant food was added to the subsoils, the amounts of 

 nitrogen nitrified were raised to 21*4, and 14*2, parts, per million. It 

 is obvious, therefore, that there would be little difficulty in accounting 

 even for the large amounts of nitrogen taken up by the Medicago saliva 

 were it established, which it certainly is not, that coincidently with the 

 deep-rooted growth, both the nitrifying organisms, and air, were 

 abundantly present in the sub-soil. 



