110 



Sir J. B. Lawes and Prof. J. H. Gilbert. 



Medicago crops, conld have had its source in nitric acid. It is obvious 

 that much nitrification takes place near the surface, but as the 

 surface-soil became even somewhat richer in nitrogen, it was clear 

 that the surface-soil has not been the primary source of the large 

 amounts of nitrogen taken up by the plants. That source must in 

 fact be 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 ? 



In another experiment, one leguminous crop, beans, had been grown 

 for many years in succession, and finally yielded very small crops, 

 containing less than 30 lbs. of nitrogen per acre. The land was then 

 left fallow for several years; barley and clover were sown in 1883, 

 and in that year, 1884, and 1885, about 300 lbs. of nitrogen per acre 

 were removed, chiefly in the clover crops- This result was obtained 

 where another leguminous crop had practically 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 con- 

 siderable depth, and where the surface was unusually poor in nitro- 

 genous 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 results adduced could leave no doubt that nitric acid was an 

 important source of the nitrogen of the Leguminosae. Indeed, existing 

 experimental evidence relating to nitric acid carries us quantitatively 

 further than any other line of explanation. But it is obviously quite 

 inadequate to account for the facts of growth, either in the case of the 

 Medicago sativa experiments, or in that of the clover on the bean- 

 exhausted land. 



Direct experiments were made to determine whether the nitrogen 

 of the Rothamsted raw clay subsoils, from which it is assumed 

 much nitrogen has 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 about - 04 or 0*05 per cent, of nitrogen, 

 and not more than 6 or 8 parts of carbon to 1 part of nitrogen, was 

 susceptible of nitrification. It was also found that nitrification was 

 more active in leguminous than in gramineous crop subsoils. Ob- 

 viously, however, the conditions of nitrification in which samples are 

 exposed in the laboratory, are very different from those of the subsoil 

 in situ. 



