38 RESULTS OF EXPERIMENTS AT ROTHAMSTED, 



EXPERIMENTS ON THE NITRIFICATION OF SOILS AND SUB-SOILS. 



It was obviously important to determine by direct experiment, 

 whether the nitrogen existing in a comparatively insoluble condition in 

 raw clay sub-soil, was susceptible of nitrification. 



It had already been found at Rothamsted, that the sub-soils of rich 

 Prairie-land were subject to nitrification ; but considering the conditions 

 of the collection and transmission of the samples in question, it was 

 considered possible that comparatively recent organic matter from the 

 surface-soil might not have been entirely excluded. Accordingly, 

 experiments were made with some of the Rothamsted raw clay sub-soils. 



The percentages of total nitrogen in the samples was determined. 

 The nitrogen as nitric acid was determined in the dry sifted soil, both 

 before and after exposure for some months under suitable conditions as 

 to temperature and moisture. After the first extraction, the soils were 

 seeded with from O'l to 0*2 gram of rich garden-soil, assumed to 

 contain nitrifying organisms, and by 0'2 grain more after some sub- 

 sequent extractions. Further, after 3 or 4 extractions, which had of 

 course removed the soluble mineral matters, a mineral mixture was 

 added. 



The results have been given and discussed in detail elsewhere, and 

 their general indication may be stated as follows : Results obtained 

 with raw, and mostly clay, sub-soils, which contain not more than 6 or 8 

 parts of carbon to 1 of nitrogen, confirm those previously obtained 

 with Prairie sub-soils containing a much higher proportion of 

 carbon, in showing that their nitrogen is susceptible of nitrification, 

 provided the organisms, and other essential conditions, are not wanting. 

 The results also consistently showed, that there is more active nitrifi- 

 cation in Leguminous than in Gramineous crop sub-soils. This, it 

 must be supposed, is partly due to more active development, and 

 greater distribution, of the organisms themselves, under the influence 

 of the Leguminous growth, with its excretions and residue, and partly 

 to the greater actual amount of such easily changeable matters, with 

 the greater amount of nitrogenous crop-residue. 



The results are also confirmed by those of experiments made in the 

 Rothamsted Laboratory by Mr. Warington, on quite distinct lines. His 

 plan was to introduce a portion of the sub-soil into a sterilized nitro- 

 genous liquid, and to determine whether nitrification took place ; the 

 result being taken to show whether or not the organisms were present 

 in the sub-soil. From his first results he concluded that in our clay 

 soils the nitrifying organism is not uniformly distributed much below 

 9 inches from the surface ; that it is sparsely distributed down to 18 

 inches, or possibly somewhat further ; but that at depths from 2 feet 

 to 8 feet, there was no trustworthy evidence to show that the clay 

 contained the nitrifying organism. 



Subsequently, he experimented with a greater variety of sub-soils, 

 and with some taken in the immediate neighbourhood of lucerne 

 roots, and gypsum was added to the sterilized liquids. 



